Abstract: In order to study the hydro-thermal aging mechanism of glass fiber (GF)/epoxy composites, the effects of hydro-thermal aging on the properties of GF/epoxy 608 (EP608) composites were investigated by gravimetric method, dynamic mechanical analyzer (DMA), SEM and vector network dielectric analyzer firstly. Then, the relationships between moisture absorption ratio, mechanical properties, dielectric properties and aging time of the composites were analyzed, and the aging mechanism was discussed. The results show that with aging time increasing, both of the mechanical properties and dielectric properties for GF/EP608 composites reduce in different degrees. The effect of hydro-thermal aging on the moisture absorption ratio of GF/EP608 composites accords with Fickian diffusion law. The plasticization, hydrolyzation of resin matrix and the failure of fiber-matrix interface are the main reasons for the decrease of the mechanical properties and dielectric properties for GF/EP608 composites. The conclusions obtained can provide scientific references for the application of GF reinforced epoxy matrix composites.