The projects of agricultural R&D investments have been evaluated largely on the basis of efficiency better than equity although both criteria are equally critical in that major fund source of agricultural R&D investments is public budget. In this study, we focus on equity aspects by identifying the effects of public knowledge created by R&D on farm income distribution using regression-based inequality decomposition method. Our major objective is to ascertain whether the R&D investments has alleviated the inequality of farm income distributions or not. Results indicate that the public knowledge stock created by agricultural R&D has significantly contributed to the reduction of the inequality measure of farm income, which implies that agricultural R&D has relatively larger positive impacts on income inequality compared to any other farm assets such as cultivated lands or intangible assets. Furthermore, we found that the contribution of agricultural R&D to the reduction of the income inequality was resulted from that the largest portion of the agricultural R&D investment portfolio has been allocated to agricultural income generating activities.