The efficacy of praziquantel was tested using 11 dogs and 3 cats infected with Clonorchis sinensis. Each experimental animal was infected with 50 metacercariae of Clonorchis sinensis respectively. The efficacy of the drug was evaluated by considerable reductions of EPG. The worms were eliminated from 91% of dogs and 100% of cats with hypodermic injection of total 75mg/kg praziquantel. This drug was effective against Clonorchis sinensis infection in dogs and cats as in the case of other trematodes.