A quantitative evaluation of the root system is necessary to discuss an interaction between plant growth and the soil environment. Although there are a number of root system evaluation methods, little studies to investigate a relationship among each method have been undertaken, particularly, between length and special distribution of the root. The current study analyzed a relationship between the Root Frequency Index (RFI), which is one of indices to indicate the spatial distribution of the plant root, and the Root Length Density (RLD) which is another index to evaluate the root length of plants. Gramineae species (i. e. Gamba grass and Maize) and Leguminous species (i. e. Stylo and Soybean) were used. While the root systems of Stylo and Soybean are characterized by the tap root system, the Gamba and Maize roots are characterized by the fibrous root system. Gamba and Stylo were established in an experimental field of semi-arid country (Thailand) having a compacted subsoil, while Maize and Soybean were subject to both a field and pot cultivation experiments. As a result, both the RFI and RLD indicated characteristics of contrasting root systems. Further, a logarithmic relationship between RFI and RLD was found independent of differences in species and soil condition. The result suggests that the root length increase exponentially as the root system develops spatially.