A laboratory incubation experiment was conducted to study the straw decomposition and soil nutrient contents after single and combined applications of maize- and horsebean straws. With the single application of maize straw, the mineralization rates of the straw and soil organic C were lower, and the immobilization of soil mineral N sustained longest. Combined application of maize- and horsebean straws promoted the mineralization of the straws organic C and soil immobilized N. Both single and combined application of the straws increased the contents of soil microbial biomass C and N significantly. It was suggested that a combined application of gramineous and leguminous straws could promote the straws decomposition and harmonize the soil nutrient supply.