The effect of surface treatment of aluminum on the bonding characteristics of aluminum-CFRP (carbon fiber reinforced plastic) composite joints has been investigated. The surface of the aluminum panel was treated by DC plasma. The plasma treatment was carried out at different treatment times and volume ratios of acetylene gas to nitrogen gas. The volume ratios used were 1 : 9, 3 : 7, 5 : 5, 7 : 3, and 9 : 1. The treatment times used were 10, 20, 30, 40, 50, and 60 s. Optimal plasma treatment conditions were determined by measuring the T-peel strength and water contact angle as functions of the treatment time and gas volume ratio. Single lap shear tests and T-peel tests were performed using plasma-treated aluminum-CFRP composite joints and regular aluminum-CFRP composite joints to determine the effect of treatment on the shear strength and T-peel strength. The results showed that the water contact angle was minimum and the peel strength was maximized when the aluminum was plasma-treated for 30 s at a volume ratio of 5: 5. The shear strength of aluminum-CFRP composite joints plasma-treated under the optimal treatment conditions was 33% higher than that of regular aluminum-CFRP composite joints. The T-peel strength of plasma-treated aluminum-CFRP composite joints was almost six times larger than that of regular aluminum-CFRP composite joints. SEM examination showed that the improvement in bond strength was attributed to a uniform spread of the epoxy adhesive due to the surface energy increase of aluminum and this resulted in a cohesive failure of the epoxy adhesive.