Improvement of production efficiency of cereal-legume intercropping could be done through fertilizer management. The objectives of this experiment were to evaluate the effect of low fertilizer rates applied to corn on biological nitrogen fixation (BNF) of intercropped soybean and the potential effect of increased BNF on intercropped corn. The experiment was conducted at the National Corn and Sorghum Research Center, Pakchong district, Nakonratchasima Province in dry season 1989/1990 and late rainy season 1990. Treatments were arranged in Split-plot in RCB with four replications. The main-plots were 15-15-15 fertilizer at the rate of 0 15 and 30 kg/rai and the sub-plots were monocropped corn (SW 3, T(,1)). monocropped soybean (SJ.4, T(,2)), soybean intrarow-intercropped into corn (T(,3)), soybean intrarow-intercropped into corn and top-dressed with 10 Kg urea/rai (T(,4)) and corn-soybean alternate row (T(,5)). Acetylene Reduction Assay was employed to estimate BNF. The results revealed that low fertilizer rates did not affect BNF, but cropping systems affected BNF significantly as T(,2) and T(,5)gave higher BNF than the others. The greatest BNF of soybean was obtained at R(,5) stage. Soybean yield was affected by low fertilizer rates and cropping system. Fertilizer rates at 15 and 30 Kg/rai increased soybean yield greater than no fertilization and soybean in T(,2) gave the greatest yield. Though fertilizer rates did increase corn yield but the differences between treatments were non-significant. High fertilizer rates tended to increase corn yield. In the dry season, corn in T(,1) and T(,4) yielded the greatest (795 and 758 Kg/rai, respectively) but in the late rainy season T(,1) gave the greatest yield (863 Kg/rai). Seed and straw of corn in T(,1) and T(,4) contained the greatest amount of nitrogen. This indicated that corn and soybean potentially competed for nitrogen. The competition was attributable to lesser nodule number per plant than normal in this experiment.