The main limitation of the use of adhesive joints is weakening the adhesive layer against damaging environmental factors such as humidity. Use of numerical methods for predicting the strength of adhesive joints exposed to moist environment can significantly save time and cost. In this study, first experimental investigation and numerical modeling of the complete process of moisture diffusion into the adhesive layer and its damaging effect on the adhesive joint strength was determined. Then this process was applied for a single lap joint of SBT 9244 pressure sensitive adhesive and AL2024-T3 aluminum alloy substrate with two 12.5 and 50 mm overlap of lengths. As the first step, moisture distribution for 30, 60 and 90 days exposure times in environmental condition of 100% relative humidity was obtained. Then single lap joint tensile test was simulated using cohesive zone model for different exposure times. In this simulation cohesive zone model parameters were determined in such a way that numerical failure load and the existing experimental failure load be in good agreement. The cohesive zone model parameters were determined dependent on the moisture content. The first simulation was done without considering swelling and in the second one swelling was considered. Swelling stress was obtained separately at different exposure time periods. It was found that swelling effect was more considerable in the joints with longer overlap length and shorted exposure time.