A real-time system is characterized by its timely response with respect to e-aernal stimuli. A timely response usually consists of a series of tasks which must be scheduled and executed accordingly so that the controlled environment can function correctly. In this paper, we present the timing constraint Petri nets (or TCPNs for short) to facilitate the modeling and analysis of real-time systems. One of the analysis capabilities of TCPNs discussed in this paper is the schedulability analysis. Through a series of modeling, analysis, and iterative refinement, the sofmtare developer can verify whether an underlying specvcation will be schedulable with respect to the given timing requirements, all via the token game played in TCPN's. Nevertheless, the analysis presented here is to analyze the schedulability of real-time systems against the imposed timing constraints, rather than to elaborate a scheduling policy.