The potential energy surfaces of the ground and valence excited states of both 3H-diazirine and diazomethane have been studied computationally by mean of the CASSCF method in conjunction with the cc-pVTZ basis set. The energies of the critical points found on such surfaces have been recomputed at the CASPT2/cc-pVTZ level. Additionally, ab initio direct dynamic trajectory calculations have been carried out on the S-1 and S-2 surfaces, starting each trajectory run at the region dominated by the conformational molecular rearrangement of diazomethane. it is found that both isomers are interconnected along a C-s reaction coordinate on each potential surface. Radiationless deactivation of the corresponding S-1 state of each isomer occurs through the same point on the surface, an S-1/S-0 conical intersection. Thereafter, the system has enough energy to surmount the barrier which leads to dissociation products (CH2 + N-2) on S-0 state. Therefore, photoexcitation to S-1 state of either diazirine of diazomethane produces methylene in its lower singlet state on a very short time scale (ca. 100 fs). Furthermore, both isomers can generate excited singlet carbene when they are excited onto the S-2 surface; in this case, they lose the activation energy passing through another common S-2/S-1 conical intersection and then proceed to dissociation into carbene and N-2 on the S-1 surface. For the special case of methylene, it rapidly experiences deexcitation to So state.