Abstract: To study the effect of glass fiber (GF) surface modified by nano-SiO₂ on mechanical properties of GF reinforced matrix composites, vacuum-assisted mold pressing (VAMP) technology was adopted to prepare polymerized poly(cyclic butylene terephthalate)(PCBT)-based composites reinforced by GF modified by various nano-SiO₂ on the surface. The effect of GF surface modification on mechanical properties of the obtained GF/PCBT composites was investigated and the hygrothermal aging resistance of GF/PCBT composites with modified GF was further studied. The fiber pull-out test results show that the interface shear strength of GF/PCBT composites enhances by 1.16 times after nano modification on fiber surface. With nano-SiO₂ content of 0.5wt%, 2wt% (mass ratio of nano-SiO₂ to PCBT), the three-point flexural strength of GF/PCBT composites increases by 1.5 times and 1.67 times, flexural modulus increases by 1.03 times and 1.17 times, respectively. Observations by SEM show that the damaged fibers are covered tightly by PCBT resin when nano-SiO₂ content is 2wt%, which is an indication of good bonding properties between the matrix and fibers. Under hygrothermal conditions, hydrone diffusion along the interphase region is hindered and results in the enhancement of the anti-aging performance of the modified GF/PCBT composites, due to the existence of nano-SiO₂ particles.