Wet oxidation behaviors of Hi-Nicalon fibers were investigated in environment Of P-H2O:O2:Ar = 14 : 8 : 78 kPa with a slow gas flow rate of 3.5 cm s(-1) above 1300 degrees C for I h. Experimental results indicated that oxidized Hi-Nicalon fibers were covered by silica. The weight gains and surface micromorphologies were strongly affected by temperature. Below 1500 degrees C, the surface of the oxidized fibers were rough-hewn and cracked, and there were no changes detected in fiber diameter. After oxidation at 1600 degrees C, the silica locally spalled and the fiber swelled in diameter. The Young-Laplace equation was applied to interpret surface micromorphologies change during wet oxidation of the specimens. The cracks in silica produced by oxidation and the microcrystal growth of the fibers at high temperature were considered for the strength degradation of the fibers. (c) 2006 Elsevier B.V. All rights reserved.