Rapid expansion of optical communication network strongly demands accurate measurements of optical fiber power. For this reason, we developed an optical fiber calorimeter as Japan’s national standard for optical fiber power and started the calibration service of optical power meters using the standard. Combining the calorimeter with a linearity calibration system enabled us to measure the optical fiber power as low as 1 µW accurately. Using these systems, we participated in international comparison of optical fiber power measurements. The results support the reliability of our calibration systems.