Millimeter wave (mmWave) wireless communication systems suffer from high path loss compared to conventional cellular communication systems. Angle of arrival (AoA) of received signal is prerequisite for beamforming technique to overcome high path loss in mmWave communications. Considering mobile environments, conventional beam estimation methods with large overhead are unacceptable for rapidly varying AoA. Therefore, beam tracking algorithm with low complexity corresponding to rapidly varying AoA is required. This paper studies on beam tracking algorithm for mmWave MIMO systems in mobile environments. We define the AoA model, and propose beam tracking algorithm using auxiliary beam pair (ABP) and extended Kalman filter (EKF). The simulation results show that proposed algorithm has better beam tracking performance than the existing methods in low signal to noise ratio (SNR) region.