Objective: To evaluate the abnormality of protein S in patients with recurrent spontaneous abortion due to antiphospholipid syndrome. Material and Method: Antigen and activity of protein S were analyzed by enzyme immunoassay and clotting method, respectively. Results: Of 18 patients with antiphospholipid syndrome, 4 patients were found to have no abnormality of protein S. There were 14 cases of protein S abnormality. Among them, there were 8 cases of type 1, 1 case of type 2, and 5 cases of type 3 protein S deficiency. Conclusion: So in the workup of patients with recurrent spontaneous abortion due to antiphospholipid syndrome, the evaluation for protein S is required.