An incubation experiment was conducted to study the distribution of ¹⁵N in mineral nitrogen, microbial biomass nitrogen and soil different size fractions after ¹⁵N labeled rice straw was added into the wushan and huangni soil. Gross nitrogen mineralization rates of rice straw in the two soils were also measured using ¹⁵N isotopic pool dilution technique. Results showed that soil mineral nitrogen decreased between 7 days and 28 days after straw was added into soils. Microbial biomass nitrogen increased before 7 days and changed less after 7 days. Mineral ¹⁵N was the highest at 7 days and accounted for 9.24%~12.3% of added straw ¹⁵N. Microbial biomass ¹⁵N was the highest at 14 days and accounted for 21.3%~40.5% of added straw ¹⁵N. Mineral ¹⁵N and microbial biomass ¹⁵N decreased between 7 days and 28 days after straw addition, which suggested that straw nitrogen was possibly lost at this time. At 56 days after straw addition, 10.5%~13.3% of added straw ¹⁵N existed in soil 53 ?m~2 mm fraction, 24.5%~26.5% was in soil 2~53 ?m fraction, and 30% was in < 2 ?m fraction. 5.7%~14.9% of added straw ¹⁵N was lost at 56 days. Straw decomposed more rapidly in the wushan soil than that in the huangni soil, and more straw ¹⁵N was lost in the wushan soil. Gross nitrogen mineralization rates of rice straw in the wushan and huangni soils were 1.61 and 1.48 mg kg^-1 d^-1 at 0.5 days and were 0.26 and 0.36 mg kg^-1 d^-1 at 56 d after straw addition, respectively.