The velocity measurement accuracy 1% from 4.06 ~475.3 mm/s and analysis of accuracy for Laser Doppler Velocimers (LDVs) based on self-mixing effect are reported. We specifically address the study of broadening of Doppler frequency due to laser spectrum, velocity distribution, speckle pattern and electronics. It can be concluded that the velocity measurement error is mainly caused by speckle patterns especially with small velocity. Extension of sampling time and selecting proper signal processing circuits is a good solution for improving velocity measurement accuracy.