Transition state theory is one of the most fundamental theories of chemical reaction dynamics,which plays a very important role in understanding chemical reaction mechanism.However,in 2004,the study of H2CO photodissociation discovered a new mechanism,named “roaming” mechanism [1],which is different from the conventional transition state mechanism.Since this discovery it has aroused widespread concern in both experiment and theory.The roaming pathway was usually found to take place on the ground state until NO3 photodissociation into NO + O2 was reported to discover the excited-state roaming mechanism recently [2].The focus of this talk introduces two systems involving excited-state roaming dynamics at the MS-CASPT2 level using the global reaction route mapping method.One is NO3 → NO + O2,where the direct dissociation on the first excited doublet state(D1)gives the minor vibrationally cold O2,and the indirection dissociation after the nonadiabatic transition to the ground doublet state(D0)produces the major vibrationally hot O2,see Fig.1(A).A series of work [2-5] in combination experimental and theoretical investigation proved that NO3 → NO + O2 proceeds exclusively by roaming.The other is CH3NH2 → CH4 + NH(X3Σ-),which involved the intersystem crossing mediated roaming dynamics [6],see Fig.1(B).