Abstract: It was possible to increase soil microbial activity and improve soil quality by using organic fertilizers. Carbon-to-nitrogen ratio is critical for high efficiency of organic fertilizer. A pot experiment was conducted to determine the effects of organic fertilizers with different carbon-to-nitrogen ratios on soil microbial biomass at the same level of nitrogen. Of the five treatments used in the study, four received organic manure in different carbon-to-nitrogen ratios (20︰1, 15︰1, 10︰1 and 5︰1) and another was non fertilizer treatment (CK). The results showed that all of the treatments with organic fertilizers increased soil contents of microbial biomass C and N. The order of soil microbial biomass C content for different treatments was 20︰1 > 10︰1 ≈ 15︰1 > 5︰1 > CK. Also the order of soil microbial biomass N content for different treatments was 15︰1 > 10︰1 > 20︰1 > 5︰1 > CK. The soil microbial biomass C/N reflected soil nitrogen biological activity. The order of soil microbial biomass C/N for different treatments was 15︰1 < 10︰1 < 20︰1 ≈ 5︰1 < CK. This suggested that organic manures with 15︰1 or 10︰1 carbon-to-nitrogen ratios improved biological activity of soil nitrogen and decreased nitrogen loss. Although soil microbial biomass was high under fertilization of organic manure with a high (20︰1) carbon-to-nitrogen ratio, biological activity of soil nitrogen was low. Soil microbial biomass was low under treatment of organic manure with a low (5︰1) carbon-to-nitrogen ratio. The study laid the theoretical and practical bases for the scientific use of organic manure in organic agriculture.