Seventy patients with clonorchis sinensis infection were divided randomly into two groups and treated with levo-praziquantel and praziquantel in the same dosage of 12.5 mg/kg twice a day for two consecutive days (total dosage of 75 mg/kg). Follow-up stool examination three and six months after treatment showed that the egg negative conversion rates with levo-praziquantel group were 92.86% and 92.59%, while those of praziquantel group were 58.62% and 53.57% respectively. The results showed that levo-praziquantel was significantly superior to praziquantel (P < 0.001). The side reactions in both group were mild and transient.