This paper presents a method for tracking human head using a particle filter to naturally integrate two complementary cues: intensity gradient and color histogram. The shape of the head is modeled as an ellipse, along which an intensity gradient is estimated, while the interior appearance is modeled using a color histogram. These two cues play complementary roles in tracking a human head with free rotation on a cluttered background. To evaluate the tracker performance, we test with both synthetic image sequences and real sequences. Experiments show that the tracker is robust to 360degree rotation of the head on a cluttered background.