The strength of adhesively bonded joints degrades when the joints are exposed to a high temperature or high humidity environment because of the degradation of the adhesive. In this paper, the strength and modulus of adhesives under hygrothermal environments were measured with respect to the absorbed moisture content and temperature. Then the strength of adhesively bonded tubular lap joints was investigated with respect to the absorbed moisture content. From the investigation it was found that the strength of the adhesives decreased as the absorbed moisture content increased, but the strength of the adhesively bonded tubular lap joints decreased less compared with that of the adhesives, due to the moisture swelling effect. Also, it was found that chemical surface treatments of the adherends improved the strength retention of the joint under a hygrothermal environment.