S:Carbon availability varies very much along soil profile and decreases from topsoil to subsoil. The effect of carbon availability index (CAI) on microbial activities in Calamagrostis angustifolia soil in the Sanjiang Plain in the Northeast China was measured. Based on the proposal about CAI and microbial respiration from Parkinson and Coleman (1999), the results showed that carbon availability limits the microbial activities in topsoil, root layer soil and subsoil initially, whereas it does not limit the microbial activity after 1.5 h incubation for recovery from the disturbance in physical, chemical and biology structure resulting from sampling, then after 5h incubation carbon availability limits microbial activity again after the labile carbon was mineralized. At the same time the soil organic matter affects the carbon availability significantly when it is lower than 10%, but little when the soil organic matter is higher than 10%. The microbial biomass carbon is linearly related to carbon availability in the Calamagrostis angustifolia soil. When the CAI is lower than 0.85, the β-glucosidase activity increases along with CAI, but decreases when CAI is larger than 0.85. S: Carbon availability varies very much along soil profile and decreases from topsoil to subsoil. The effect of carbon availability index (CAI) on microbial activities in Calamagrostis angustifolia soil in the Sanjiang Plain in the Northeast China was measured. Based on the proposal about CAI and microbial respiration from Parkinson and Coleman (1999), the results showed that carbon availability limits the microbial activities in topsoil, root layer soil and subsoil initially, whereas it does not limit the microbial activity after 1.5 h incubation for recovery from the disturbance in physical , chemical and biology structure resulting from sampling, then after 5h incorporated carbon availability limits that microbial activity again after the labile carbon was mineralized. At the same time the soil organic matter affects the carbon availability significant when it is lower than 10%, but little when the soil organic matter is higher than 10%. The microbial biomass carbon is linearly related to carbon availability in the Calamagrostis angustifolia soil. When the CAI is lower than 0.85, the β-glucosidase activity increases along with CAI, but decreases when CAI is larger than 0.85.