The dissociation dynamics of ClN3 have been studied by ab-initio classical trajectory and static potential energy surface calculations. The energies and structures of molecular isomers and transition states are obtained from optimization. Three isomers and five transition states of ClN3 are identified, and it is confirmed that the cyclic-N-3 can be generated directly dissociation from one of isomers and from bending linear-N-3 after dissociation. The classical trajectories simulations provide the ultrafast formation dynamics of cyclic-N-3 during the dissociation, and the calculation of dissociation dynamics of ClN3 provides us a new insight on the pathway of formation cyclic-N-3 from molecules.