Managers’ capabilities and performance are the determinations of a firm’s prosperity. Hence, how to enhance managers’ capabilities to promote their work performance is an important part of a firm’s policy of human resources development. Therefore, this study will explore the relationships among “Self-Directed Learning”, “Managerial Competency” and “Job Performance”. In addition, scholars of the theory of organizational Justice argued that the organizational justice perceived by its members can exact significant impacts on the members’ work motivation, performance, attitudes and behaviors. We should consider the organization’s environment when we intend to promote the managers’ capabilities. Therefore, this study also explores the moderating role of “Organizational Justice” on the relationship between “Self-Directed Learning” and “Managerial Competency”. A questionnaire survey was conducted to test the study’s hypotheses. Copies of the questionnaire were sent to 220 managers of domestic companies and 139 effective questionnaires were received. This results in an effective response rate of 63.2%. After analyzing the survey data and testing the hypotheses, the study finds: 1. Confirmatory factor analysis (CFA) was used to test the research variables. After deleting a few items, all the fitness indices of the research variables are within the recommended ranges and indicate that all the measure models of the research variables are adequate. In summary, “Self-Directed Learning” has four constructs including “learning preference”, “active learning”, “initiative in learning” and “motivation of learning”; “Managerial Competency” has four constructs including “competency of conceptualization”, “interpersonal skill”, “administration skill” and “problem-solving skill”; “Job Performance” has two constructs including “effectiveness” and “quality performance”; and “Organizational Justice” has two constructs including “distributive justice” and “procedure justice”. 2. Hypothesis 1 (“Self-Directed Learning” has positive impacts on “Managerial Competency”) is only partially supported. Among the four constructs of “Self-Directed Learning”, only “initiative in learning” has positive impacts on all the four constructs of “Managerial Competency” (namely, “competency of conceptualization”, “interpersonal skill”, “administration skill” and “problem-solving skill”); while “active learning” has positive impacts on “interpersonal skill” and “problem-solving skill” but has no impacts on “competency of conceptualization” and “administration skill”; “learning preference” has impacts on “interpersonal skill” but has no impacts on the rest three constructs of “Managerial Competency”; and “motivation of learning” has no impacts on all the four constructs of “Managerial Competency”. 3. The results of testing of Hypothesis 2 (“Managerial Competency” has positive impacts on “Job Performance”) indicate that: among the four constructs of “Managerial Competency”, “interpersonal skill” has positive impacts on the two constructs of “Job Performance” that are “effectiveness” and “quality performance”; both “competency of conceptualization” and “problem-solving skill” have positive impacts on “effectiveness” but has no impacts on “quality performance”; and “administration skill” has no impacts on the two constructs of “Job Performance”. 4. The results of testing of Hypothesis 3 (“Organizational Justice” has moderating effects on the relationship between “Self-Directed Learning” and “Managerial Competency”) indicate that: the two constructs of “Organizational Justice” (namely, “distributive justice” and “procedure justice”) have weakening effects on the positive impacts of “initiative in learning” on “competency of conceptualization” respectively; while “distributive justice” has weakening effects on the positive impacts of “active learning” on “interpersonal skill”. The results of this study may have some managerial implications. For example, because the negative moderating effects of “Organizational Justice” on the relationship between “Self-Directed Learning” and “Managerial Competency”, enterprises should co-opt managers with high level of self-directed learning propensity and high level of AQ (Adversity Quotient) for potential enhancement of their managerial competence in the future.