Abstract Novolac epoxy resins, with the high glass transition temperature (Tg) and excellent mechanical strength, are widely applied in the molding and sealing compounds for the production of electronic devices. However, weather exposure and environmental elements are inclined to affect their durability severely. This work focuses on the hygrothermal aging process and mechanism of the novolac epoxy resin. The effect of humidity and time in hygrothermal aging on structural and mechanical properties of the novolac epoxy resin was deeply studied. The moisture absorption increases linearly with the square root of aging time, and it follows the Fick's second law. There are two main categories of reactions in hygrothermal aging: the first one is the post curing process, which leads to a larger crosslinking density and a reduced interior stress; while the other is the plasticization and deterioration of epoxy resin attributed to moisture ingress. The combination of above factors leads to a decrease-increase-decrease variation in mechanical properties. This work is believed to benefit the wide and safe application of a certain novolac epoxy resin system in engineering application.