Running regularly is a good way to keep fit. However, people's laziness usually stop them from keep up. Motivated by the competition and achievement system in many video games, we propose Runking, a mobile social network based persuasion system for running exercise. Runking can rank users' exercise records among friends and push those who are lazier than others. Meanwhile, if users keep on active, different achievements can be unlocked, which can further incent users to exercise regularly. The system is build upon smartphones. By utilizing sensory data from smartphones, it can track and measure people's daily running record without human interaction. Evaluations show that the system has a high recognition rate for identifying users' activity and can reduce the network communication by using disconnected operations. Therefore, it can help users improve their fitness effectively.