A method of tracking target object includes following steps of: controlling a following robot to capture a target object for obtaining a comparison image; determining in the comparison image a plurality of sampling particles corresponding to the target object; calculating a predicted position of each sampling particle; moving toward a predicted direction corresponding to the predicted positions for following the target object; re-sampling the sampling particles according to a difference between a reference image and the comparison image; configuring the compare image as the new reference image; and, performing above-mentioned steps repeatedly until the following robot stops following the target object. Therefore, the analysis efficiency and stability of tracking effectively can be improved; the real-time following and the following robot operate without a signal transmitter can be achieved as well.