Adhesive bonding techniques have been widely used and studied in automobile industry, enabling to join dissimilar materials, such as carbon fiber reinforced plastic (CFRP) composites and metals. However, durability of an adhesive joint is often a major concern in the severe service conditions of vehicle due to the nature of polymeric materials involved. This study aims to investigate the failure mechanisms of CFRP/Aluminum adhesive joints after certain hygrothermal degradation. In addition to mechanical strengths, the failure process and failure modes of the aged joints were studied herein. The results divulged that the strengths of adhesive joints decreased after hygrothermal degradation but had a noticeable increase after 40 days of aging. The failure process after 20 day aging was different from that without aging. In terms of variations in residual strengths and failure process, the failure modes exhibited three different types overall and two uncommon failure modes were identified. It is shown that the residual mechanical properties of adhesive material were a result of the combined effects of water plasticization and temperature post-curing. Its moisture absorption characteristics is found to be related to its residual mechanical properties. The study is anticipated to gain some insights into the hygrothermal effects on the residual properties of the CFRP/Al adhesive joint.