The Japan Calibration Service System (JCSS) organized in 1993 accredits the measurement capability of calibration laboratories and ensures the traceability to the national measurement standards. As an essential part of accreditation of the measurement capability of calibration laboratories for the weights, the International Accreditation Japan (IAJapan) of National Institute of Technology and Evaluation has been operating the JCSS proficiency testing programs with the technical support of the National Metrology Institute of Japan (NMIJ/AIST). Up to now, five proficiency testing programs have been carried out for the JCSS weight calibration laboratories in the range of 2 mg to 10 kg. The proficiency testing programs organized by the IAJapan were carried out in accordance with ISO/IEC Guide 43 (JIS Q 17043), and the NMIJ was responsible for the technical aspect as a reference laboratory. This paper describes the methods of the five proficiency testing programs during the period from 1997 to 2009, and outlines assessment of the measurement capability of the JCSS weight calibration laboratories.