In the present work, the combined electronic structure calculations and dynamics simulations have been performed to explore the decomposition reactions of 2H-azirine, 2-phenyl-2H-azirine, and 3-phenyl-2H-azirine in gas phase. Thermal cleavage of the C-C single bond yields nitrile ylides as products with a high barrier (>50.0 kcal mol(-1)). On the other hand, photochemical cleavage reactions starting from the (1)n pi* state of 2H-azirine and 3-phenyl-2H-azirine are ultrafast nonadiabatic processes (<100 fs), leading to nitrile ylides through the S-1/S-0 conical intersection. The fast formation of ylides in experiments was well reproduced by present dynamics simulations. For 3-phenyl-2H-azirine, population of the S-1((1)pi pi*) state is another decay pathway for the S-2((1)n pi*) state. The C-N bond cleavage upon photoexcitation was usually considered to take place in T-1 state. However, our calculations reveal that photocleavage of the C-N single bond for 2-phenyl-2H-azirine is very likely to take place in S-1 state. The present work provides new insights into photocleavage mechanism of 2H-azirine and the related derivatives.