Abstract: In the current study, the effects of ryegrass (Lolium multiflroum Lam.) planted as a winter cover crop on soil microbial biomass C and N in six soil types were evaluated by a chloroform-fumigation-extraction method in southern China’s double-rice region. According to the results, soil type and sampling date have significant effects (P＜0.05) both on soil microbial biomass C and N. In the six different soil types the highest soil microbial biomass C was found in Yellow clayey soil (YCS) and the lowest value in Alluvial sandy soil (ASS), values averaged across the year were 450.8 and 60.8 mg kg-1, respectively. When arranged in a decreasing order, it was Yellow clayey soil (YCS) ＞Purple clayey soil (PCS)＞Granitic sandy soil (GSS)＞Red clayey soil (RCS)＞Grey clayey soil (GCS)＞Alluvial sandy soil (ASS). Ryegrass increased soil organic matter and microbial biomass C and N significantly (P＜0.05) compared with CK(fallow). All the treatments followed a similar seasonal trend in soil microbial biomass: greater values were found at maturity stage and at middle-rice season while the lower values were found at tilling stage and at early-rice season. We thus concluded that soil microbial activities were related to crop stages and the microbial biomass was affected by a complicity of factors including soil type, nutrient availability and crop management. Ryegrass could increase soil organic matter and microbial biomass significantly, especially in soils with a high organic degradation rate and low available nutrient contents.