In the present work, density functional theory (QM) and molecular mechanics (MM) method were used to study mechanistic photodissociation of CO-ligated neuroglobin (Ngb-CO). It was found that all the electronic states investigated here are bound with respect to the Fe-CO separation, except for a couple of near-degenerate states (E-1) that are repulsive. Irradiation of Ngb-CO at 533 nm leads to the system in the lowest two excited singlet states ((1)Q), where non-adiabatic CO dissociation proceeds with high efficiency through the intersection between (1)Q and E-1. Soret band -(B-1) is the strongest in the absorption spectra of Ngb-CO with the peak at 415 nm. The systems in the B-1 states decay to the E-1 states via fast internal conversion, which is followed by the CO dissociation. The CO dissociation induces a considerable change in the structure of the Ngb protein. The initial dissociation involves a rotation of CO, which is accompanied with movement of several residues. When the Fe-C distance is larger than a critical value of 3.0 angstrom, the CO molecules transfer more freely into the cavity of the protein. The pentacoordinated heme was found to be a transient intermediate after CO dissociation.