Birch (Betula platyphlla) and poplar (Populus davidiana) are the typical heliophilous, fast growing species and frequently form the pure birch forest (BF) or birch and poplar mixed forest (BPF) in North China. A reciprocal litter bag experiment was carried out to investigate the leaf litter decomposition process in BF and BPF and the effect of forest types on litter decomposition using pure birch leaves litter and the mixed litter of birch, poplar and ash (Fraxinus mandshurica) leaves. The results of the two year decomposition showed that the effect of forest types and litter type on litter decomposition was not significant at the fast decomposing stage but at the slow decomposing stage. The environmental conditions of BPF contribute to the nutrient release and decay of the two litter types. Compared with birch leaves, mixed leaves had a significantly higher substrate quality and showed a substantially faster decomposing rate and nutrient release rate. Litter quality to some degree can mask the effect of forest type on litter decomposition. In conclusion, BPF exhibits a faster carbon and nutrient cycling rate than BF does.