In this study, we constructed a control chart for C(subscript pm) with variable sampling interval. It was found that the proposed control chart are more efficiency to detect the small shift than the Shewhart-type control chart and the appropriate short and long sampling intervals are set to be 0.1 and 1.9 respectively. This control chart can be applied to the situation where a company wants to ensure the quality of the process capability of products from other company.