Carbon fiber reinforced thermoplastics (CFRTP) using polyamide 6 (PA6) as the matrix resin were examined for the effects of ozone oxidation treatment and water absorption on the mechanical properties. Strength tests were conducted on CFRTP samples subjected to vacuum drying, conditioning, and temperature and humidity cycles. The relationship between the water content and the mechanical strength was clarified from the results. The water content of the samples was measured using the Karl Fischer method. In comparison with the untreated sample, the CFRTP sample subjected to ozone oxidation treatment (for both the CF fabrics and the PA6 films) exhibited higher values of flexural and tensile strengths at all the environmental conditions. This is attributed to the improved interfacial adhesion between the CF and the PA6 in the ozone-oxidized CFRTP sample even after water absorption. However, it was found that the rate of increase due to the ozone oxidation treatment tended to reduce under increased water penetration, because the interfacial adhesion between the CF and the PA6 matrix decreased.