The chemical stability of perhydropolysilazane is improved by the introduction of HMDS (1, 1, 1, 3, 3, 3-hexamethyldisilazane) radical. The increase rate of the number average molecular weight of HMDS radical-introduced perhydropolysilazane was ten times slower than that of raw perhydropolysilazane. H-1-NMR analysis revealed that HMDS molecules mainly reacted with terminal Si-H-3 groups of perhydropolysilazane.