In this paper, two language characterizations for weak liveness (free-deadlock) and liveness of Petri nets are given. Some language properties of synchronous composed Petri nets are discussed. Based on Petri net language, a necessary and sufficient condition is given for live Petri net (bounded), and then the liveness presevation in a synchronous composed net is studied and a necessary and sufficient condition is obtained. Those results give a formal language method for net liveness testing and liveness controlling.