Multimedia Messaging Service (MMS) is an interactive communication process between people. So,we treat this network as a social network. In this paper we analyze user behaviors of MMS system. Using a province's dataset of China Mobile, we obtain almost entirely of the messages has been sent via GSM；VAS applications were very popular and profitable；The size and the storage time of message is very important for a MMS system；The in-degree and out-degree distribution both obey power law form. A big found is that users have acknowledgement effects between each other.