A vermifilter system packed with quartz sands and ceramsite was studied for domestic wastewater treatment. Results showed that good performance of vermifilter was achieved and removal rates were COD (47.3 – 64.7%), BOD5 (54.78 – 66.36%), SS (57.18 – 77.90%), TN (7.63 – 14.90%), and NH4-N (21.01 – 62.31%), respectively. An increase in hydraulic loading led to a decrease in treatment efficiency and adult earthworm abundance. In addition, activities of protease, alkaline phosphatase (ALP), and cellulase in earthworm body dropped, but superoxide dismutase (SOD) and catalase (CAT) increased with the hydraulic loading. Correlation analysis implied that larger earthworm (>0.3 g) abundance might play more positive role on wastewater treatment in vermifilter, compared to smaller worm. Earthworm enzymatic activities had significant correlation with treatment efficiency of COD and BOD5 by vermifilter. Thus an important relationship exists for earthworm population dynamics and enzymatic activities with COD and BOD5 removal rates of domestic wastewater by vermifilter. Key words: Vermifilter, wastewater, earthworm, population dynamics, enzymatic activity, treatment efficiency.