Abstract: Two factors affecting accuracy of transit time in ultrasonic liquid flow measurement are ana-lyzed,and the theoretical derivation is made.From the single path model,the principle and the existing problem in transit time measurement are presented.Then,the effect of system SNR on transit time mea-surement error in white Gaussian noise model and the effect of threshold on transit time measurement error while using zero-crossing and threshold detector are discussed.The simulation result with figures showing the above discussed effects is given.