A critical distance based method was proposed for predicting the strength of adhesive single lap joints. Using this method, the failure of SLJs was predicted when the longitudinal strain along the adhesive mid-plane reached a critical value at a specific critical distance. The two parameters of the method including the critical longitudinal strain and the critical distance can be determined using experimental results. Several single lap joints with different overlap lengths and substrate thicknesses were manufactured and tested under quasi-static loading. It was found that the critical distance was independent of the overlap length and the substrate thickness while the critical longitudinal strain was found to be dependent on the substrate thickness. However, the effect of substrate thickness on the critical longitudinal strain decreased by increasing the substrate thickness. The correlation between the experimental and predicted failure loads was found to be very well.