Modern smart phones are equipped with a wide variety of sensors including GPS, accelerometer, gyroscope, or even biometric sensor. Many intelligent context-aware applications are thus also become popular. In particular, by using GPS data, many Location Based Services (LBSs) have been developed and deployed. In the literature, analysis based on the collection of GPS data from a large amount of users has also been widely studied, such as user classification, location prediction, personalized travel route recommendation, etc. However, analyzing the swarm behavior of a group of users based on their GPS data has received little attention. In this paper, we perform both quantitative and qualitative analysis on the swarm behavior of a group of users based on the Boids model proposed by Craig Reynolds. Specifically, we are interested in discovering the features of Boids model by analyzing the GPS trajectories of a group of people. Our results confirm that the moving of a group of people exhibits flocking behavior and can be explained by the features of Boids model.