The reproductive ecology and mating success of male Limnonectes kuhlii, one of the fanged frogs which are characterized by a suite of unusual sexually dimorphic traits (males have larger body, head, and fang sizes than females), were studied under natural conditions in subtropical Taiwan in order to clarify the role of sexual selection on male body size. There were no significant temporal peaks in the number of adult males or amplexed females observed along an 81-m transect of a small creek throughout a 3-mo study period. The nightly sex ratio was highly male biased. Over the study period, the distribution pattern of males showed a significant tendency toward clumping at oviposition sites (represented as oviposition sections of the creek where females laid eggs). During the nights, males usually showed a nonclumped distribution pattern along the transect, probably due to male-male interactions mediated through vocalization and physical combat, including vigorous biting. The calling sites of males were considered as breeding territories and sites of oviposition. Larger males often changed their nightly position along the transect, whereas smaller males remained at the same sites over the study period. The number of different oviposition sections in which each male was present over the study period ranged from 0–11 and was positively correlated with male body size. The number of matings by each male ranged from 0–9, and 42% of the males mated at least once. Male mating success was positively correlated with six male traits that included body size and the number of nights present. However, multivariate analysis revealed that a direct factor determining male mating success was the number of different oviposition sites which a male visited. This study also demonstrated that large-male mating advantage in L. kuhlii occurred indirectly through the size-dependent spatial movement pattern. A large body size in male L. kuhlii seems to have evolved through intrasexual selection (male-male competition by physical combat) in the context of a resource-defense polygyny mating system.