The characteristics of the decomposition and the nutrient return of the leaf litters under the pure and mixed plantations of Juglans mandshurica and Larix gmelinii were studied by decomposing-bag method.The result showed that in one growth season the decomposing rates were as follows:the J.mandshurica pure plantation>mixed plantation>the L.gmelinii pure plantation.The return rate of the nitrogen element was the fastest in the pure plantation of J.mandshurica and on the other hand the return rates of the phosphorus and potassium were the fastest in the pure plantation of L.gmelinii.The return rate of the nitrogen in the mixed plantation was obviously faster than it was in the L.gmelinii pure plantation and those of the phosphorus and potassium were obviously faster than they were in the J.mandshurica pure plantation.In contrast with the pure plantations,the improvement of the return rates of the nutrient elements in the mixed plantation may be one of the machenisms for the growth improvement of the mixed plantation.