Steel fibers are widely used in cementitious composite materials to enhance their mechanical properties, such as tensile strength and toughness. However, the effectiveness of these fibers largely depends on their surface characteristics and bonding with the cement matrix. This study investigated the effects of various treatment processes on the microhardness and mechanical strength of steel fibers in cementitious composite materials. These methods include acetone and acid washing, silane coupling agent treatment, and nanosilica coating. Fibers washed with acetone exhibited a cleaner surface, primarily due to the removal of impurities. Acid treatment resulted in a notably roughened surface, which significantly enhanced mechanical interlocking with the surrounding matrix. Silane treatment led to an uneven surface with distinct vertical textures, potentially improving adhesion properties. Meanwhile, fibers treated with nanosilica displayed a coating of nanoparticles adhering to the surface, which may further influence the fiber–matrix interaction. The results of the mechanical properties tests indicated that nanosilica coating was the most effective in improving both the flexural and compressive strengths, especially in the early strengths in the cement matrix.