[en] A Field experiment was conducted at the Chiang Mai field crops research center to study the response and nitrogen fixation of soybeans on nitrogen fertilizer supplementation at early pod-filling stage. Two soybean genotypes SJ5 and NW1 were used in this study. Experimental design was split plot in RCB. Supplemental nitrogen was added at pod-filling stage in the rate of 0, 3, 6 and 9 kg N/ria. ¹⁵ N labelled ¹⁵(N H₄)₂ SO₄ was added for 4 plants per treatment in the 3 x 5 m² subplot. The harvesting area was 2.2 x 4.0 m². The cultural practices followed the recommended standards of the farmer, by mixing the seeds with Rhizobium inoculant. Basal fertilizer was added at the rate of 3 - 9 - 6 (N - P₂ O₅ - K₂ O) kg/ria at planting. The results showed that soybean NW1 seed yield increase 26% to the addition of supplemental nitrogen at 3 kg N/ria whereas SJ5 gave no response for all 3 rates of nitrogen addition. There was a significant response in both genotypes in terms of seed protein. The seed protein for both genotypes increased with the incremental addition of supplemental nitrogen at rate of 9 kg N/ria, average about 3.6%. ¹⁵ N isotope dilution method indicated that no fixation occurred with N supplementation for both genotypes. But the percent fertilizer use efficiency (% FUE) significantly decreased with rates of N supplementation. The % FUE of NW1 was lower than SJ5 due to the short maturity. Total N yield for each vegetative part of SJ5 was no significantly different with the three rates of N addition. Total N yield in grain of NW1 increased with increasing of supplemental nitrogen. It is therefore concluded that nitrogen applied to the plant at pod-filling is the growth factor, to promote production and seed quality of NW1 in Chiang Mai