An equipment including aircraft engine simulated environmental system and sample loading stage was manufactured for annealing Hi-Nicalon monofilament fibres in corrosive environments. The fibres were exposed in air and 14% H 2 O/8% O 2 /78% Ar atmospheres at 1200°C for 1–100h, respectively. Experimental results indicate that tensile strength decreases, whereas oxide layer thickness and grain size of SiO 2 increase with rising annealing time. The specimens show brittle/flat fracture, most of cracks initiate at the interface between SiC and SiO 2 region of fibres. The control effect of water vapor on formation and structural evolution of the passive film was discussed.