In this study, it was proposed that whispering gallery mode (WGM) resonances was applied for diameter measurement of microsphere. WGM resonances were excited in a microsphere by introducing light through tapered fiber that thinned optical fiber to a few micrometers. Resonance wavelengths could be measured by monitoring intensity of the transmitted light of tapered fiber, since the light is strongly coupled to the microsphere at the resonance wavelengths. Therefore it is important to address the coupling conditions in order to measure WGM resonances accurately. Thus, optimization of the coupling is key issue to determine the diameter of a microsphere. Numerical analysis was implemented to estimate the coupling coefficients of each modes of WGM. Then the coupling was conducted experimentally as parameter was distance between the sphere and the tapered fiber.