Abstract: An incubation experiment was carried out to reveal decomposition characteristics of straw of maize and wheat and their effects on soil carbon and nitrogen contents. The experiment was lasted in dynamic microcosms for 32 days at 30℃ with 8 treatments combined with 2 moisture levels, namely, relative water content of 60% (M60) and 80% (M80), and 4 straw levels, namely, maize straw addition, wheat straw addition, mixed addition and no addition. The results show that，1) there is a strong influence of soil water content on the decomposition of the both crop straw in soils, and the rate of CO2 evolution at RCW 60% is lower than that at RCW 80% throughout the incubation period. Moreover, the ratios of CO2-C derived from straw to the total released C from straw are 40.1% and 51.5% under the M60 and M80 treatments, respectively, and the increases of SOC content are 2.24 and 1.43 g/kg for the two treatments. 2) The rate of CO2-C evolution from maize straw decomposition is consistently higher than that from wheat straw. In the treatments where maize straw are added, the cumulative amount of CO2-C evolution and net SOC increase are 408.35 mg/pot and 2.12 g/kg, and those for wheat straw additions are 378.94 mg/pot and 1.56 g/kg. When the two types of straw are mixed with soil, the two values range between those of solely additions. 3) Compared to no straw addition at the end of 32 d incubation, the contents of SOC, microbial biomass C, total N and microbial biomass N in the straw addition treatments are significantly increased, and they are almost ranked in maize straw＞maize and wheat straw＞wheat straw. Therefore, it is more easily to transform the straw carbon into inorganic C (CO2) than into SOC under high water condition, and maize straw decomposition is easier than that of wheat straw. In conclusion, the high contribution of straw returning to soil can be regarded as a supplement to soil carbon pool and nutritious elements including N, and microbe, and thus soil quality is improved.