To solve the problem of the track initiation typically encountered in the presence of clutters, missed detections and unknown number of targets, a track initiation algorithm for a passive sensor system is presented. First, a part of false intersection points is first eliminated by some heuristic rules, then the observation space is divided into uniform-size grid by using probabilistic grid concept, and the probability of the measurements falling within each grid is calculated. Finally, the tracks are detected by using the serial Hough. Simulation results show the presented algorithm can initiate tracks effectively. Besides, the algorithm reduces the ratio of false track initiation.