γ- Hexanitrohexaazaisowurtzitane (HNIW) was obtained with the yield of more than 88% and the purity of larger than 99.4% by nitration of tetraacetydiformylhexaazaisowurtzitane (TADFIW) in 82.48%-88.89% nitric acid. The results show that the optimal concentration of nitric acid is 86.66% as the yield is 94.0% and the purity is 99.51%. The main impurity pentanitromonoacetylhexaazaisowurtzitane （PNMAIW）in HNIW prepared by this method was seperated and characterized, that is different from the main impurity pentanitromonoformylhexaazaisowurtzitane （PNMFIW）in HNIW from nitration of TADFIW in the mixture of nitric acid and sulfuric acid. It shows that nitration mechanisms in those nitrating environments are different.