This paper presents a method, namely mmTracking, for device trajectory tracking in a millimeter wave (mmWave) communication system. In mmTracking, the base station (BS) relies on one line-of-sight (LoS) path and at least two non-line-of-sight (NLoS) paths, which are reflected off two walls respectively, of the uplink channel to track the location of a mobile device versus time. There are at least three radio frequency (RF) chains at the BS. Analog phased array with narrow and adjustable receive beam is connected to each RF chain to capture one signal path, where the angle of arrival (AoA) can be roughly estimated. Due to the carrier frequency offset between the transmitter and the BS, the Doppler frequency of each path could hardly be estimated accurately. Instead, the differences of Doppler frequencies of the three paths can be estimated with much better accuracy. Therefore, a trajectory tracking method based on the Doppler difference and AoA estimations is proposed in mmTracking. Experimental results in a typical indoor environment demonstrate that the average error of transmitter localization and trajectory tracking is less than 20 cm.