AM1 and PM3 semi-empirical methods were used to conduct theoretical studies on possible polymorphs of pentanitromonoformylhexaazaisowurtzitane (PNMFIW), and a close link between PNMFIW and Hexanitrohexaazaisowurtzitane (HNIW), especially in sensitivity, is shown. The optimized geometries of possible polymorphs of PNMFIW are similar to those of HNIW. PNMFIW in epsilon-HNIW prepared from tetraacetyldiformylhexaazaisowurtzitane is predicted to have a D-form. The average N-N bond lengths of PNMFIW computed by AM1 and PM3 methods are shorter than those of HNIW. The differences in energy and thermochemistry values between PNMFIW and HNIW are insignificant except molecular energies 255.750(.)mol(-1) for D-form PNMFIW and 460.36 kJ(.)mol(-1) for E-HNIW. Based on a Mulliken population analysis of the N-N bonds, the impact sensitivities of A-, B-, C- and D-forms of PNMFIW are estimated to be lower than those of the corresponding polymorphs of HNIW. Taking into account all N-N bond lengths and overall molecule size, the shock sensitivities of all forms PNMFIW are predicted to be almost the same, and lower than those of HNIW.