Bhattacharyya coefficient(BC) is always utilized as the measurement of similarity between models of object and candidate region in particle filter visual tracking based on color histogram. This paper analyzes and explains that there exists thousands of peaks of BC of internal regions in object such that particle filter tracking with BC only adapts to object shrinking while not to object dilating. Therefore, a modified version of BC is proposed, which is proved to be single-peak, and the particle filter based on which is applicable for both object shrinking and dilating. Both analysis and experimental results demonstrate the robustness and accuracy of particle filter tracking based on the modified BC.