Displacement estimation in ultrasound is a common yet important task for various applications, including tissue/blood motion, elastragraphy imaging, temperature estimation, etc. Due to the discrete feature of window-based cross correlation, an accurate subsample displacement estimator is in great demand with higher and higher frame rate. In this paper, we present a zero phase crossing method based on complex cross correlation. The novelty of this technique is twofold. First, the phase of complex cross correlation is linear. Second, the subsample of maximum magnitude of complex correlation lies in zero phase line. Therefore, subsample displacement can be easily estimated by finding the zero phase crossing. Simulation results shows that this method has a better performance than other estimators, including parabolic fitting, cubic fitting, cosine fitting, etc, in terms of bias and standard deviation under various conditions. The computational cost of this technique is also considered for real-time implementation.