When measurement of reverberation time is done in a test chamber to evaluate acoustical property of materials, spatial averaging of reverberation time should be done. Using microphone rotator is recognized as one of the tool to do spatial averaging. This study compares between three methods of spatial averaging of reverberation times measured in small rectangular test chamber (3m x 4m x 5m). The first method is averaging reverberation time measured at 5 of fixed position used as standard positions for testing, the second is at 72 fixed positions on the circle of microphone rotator, and the third is with microphone rotator (64 s/rotation). The result of comparison between three method revels that reverberation time measured by rotating microphone has more scatter than and presents different reverberation time from those measured by other method especially at lower frequency bands. Simulation of microphone rotation with the decay curves measured at 72 fixed positions suggests that spatial distribution of steady state sound pressure level, rotation speed of microphone, and reverberation time of test chamber are key factors of errors. As a conclusion, the strict guideline for measurement of reverberation time with microphone rotator should be presented to minimize errors.