Ultrafast intersystem crossing mechanisms for two p- and m-nitrophenol groups (PNP and MNP) have been investigated using ab initio nonadiabatic molecular dynamics simulations at the 6SA-CASSCF level of theory. Trajectory surface hopping simulation has been performed within an intersystem crossing network constructed from two low-lying singlets (S 0 and S 1 ) and two low-lying triplets (T 1 and T 2 ). It is found that the dominant relaxation S 1 → T 2 pathway accounts for 65.4% (85.0%) of the quantum yield with a time constant of 13.4 fs (22 fs) and the S 1 → T 2 → S 0 pathway accounts for 33.1% (13.5%) with a time constant of 275 fs (375 fs) for PNP (MNP). In comparison with the previously studied excited-state proton transfer process for ONP, the dominant relaxation S 1 → T 2 → T 1 pathway accounts for 49.3% with a time constant of 40 fs and the S 1 → T 2 → T 1 → S 0 pathway accounts for 47.5% with a time constant of 300 fs. The relaxation mechanisms and electronic structures of the intersystem crossings are in close relation with the relative motion between the torsion motions of the nitro-group and the hydroxyl group. The present simulation provides new physical insight for understanding ultrafast photochemical intersystem crossing dynamics.