Application of chemical fertilizer incorporated with rice straw can improve soil fertility and N supply. In most of such experiments, however, attention was mainly paid to the function of chemical fertilizers while the effect of the straw N on the transformation of N from chemical fertilizers was to certain extent neglected. In order to investigate the effect of rice straw application on the transformation and residue of N from different fertilizers, pot experiments with two different types of soils (Reddish yellow clayey soil, 1 ∶ 1 type, low fertility level and Purple alluvial clayey soil, 2:1 type, high fertility level) were conducted by applying the ¹⁵N cross labeled technique with urea and rice straw as the N sources. The designed experiments included four treatments and 8 replications (total 64 pots including pots for sampling): the control (CK with no N application), labeled urea (¹⁵NU), combination of labeled urea and rice straw powder (¹⁵NU-S) and the combination of labeled rice straw power and urea (¹⁵NS-U). The results obtained from the experiments indicated that the uptake rate of the soil native N by rice exceeded 60% while that of the applied N in the low fertility level soil (Reddish clayey soil) was higher than in the high fertility level soil (Purple alluvial soil). During the rice growth period, the assimilation percentages of the labeled urea N by microorganism in reddish clayey and purple alluvial soils were 1.8%-8.3% and 1.8%-19.2%, respectively, while those of the labeled rice straw N in these two types of soils were 1.7%-5.0% and 2.0%-6.2%, respectively. The fixation percentages of the labeled urea N by soil clay were 0.3%-2.1% and 3.5%-18.7% while those of the labeled rice straw N were 0.2%-0.9% and 1.7%-5.0% in reddish clayey and purple alluvial soils, respectively,. The residue percentages of the labeled urea N of ¹⁵NU in reddish clayey soil and purple alluvial soil and were 14.5% and 16.9% respectively; while those of ¹⁵NU+S in both soils were 17%. The residue percentage of the rice straw N in reddish clayey soil and purple alluvial soil were 38.8% and 41.5%, respectively. In conclusion, the combined application of urea and rice straw increased the assimilation rate of urea N by microorganism and decreased the fixation level of urea N by soil clay. In addition, the combined application increased both the N uptake rate by crops and the residue quantity of organic N from exogenous N associated with decreased inorganic N residue from exogenous N.