<p>This study investigated the changes in microstructure and mechanical properties of C/SiC composites fabricated by a liquid silicon infiltration (LSI) process, utilizing carbon fibers surface pre-treated with Ar plasma. Analysis of the carbon fiber surface structure revealed that T300-grade and T700-grade carbon fibers possessed distinct initial surface structures, and applying surface pre-treatment under identical conditions resulted in differing surface structural deformations. Through cross-sectional images of each C/C carbonized preform, the crack spacing between fiber bundles and the matrix was analyzed. It was found that for T300-grade fibers subjected to surface pre-treatment, the crack spacing increased by up to 5.5 times compared to the untreated state, whereas for T700-grade fibers, it decreased to approximately 22% of the original level. Consequently, T700-grade C/SiC composites with surface pre-treatment exhibited a significant reduction in residual Si within the fiber bundles and matrix, leading to an improvement in bending strength by up to 59.2% compared to untreated samples. Conversely, for T300-grade carbon fibers, a large amount of macroscopic residual Si was formed, resulting in a 15.7% decrease in bending strength. These variations in C/SiC strength are attributed to changes in the carbon fiber’s surface bonding strength induced by surface treatment, and subsequently, to differences in residual Si formation stemming from changes in crack spacing between the fiber bundles and the matrix.</p>