Fatigue tests of the single lap adhesive joints of thin mild steel plates were carried out and the effect of the thickness of the adhesive and plate on the fatigue strength was investigated. The boundary element two-dimensional elastostatic analysis was also carried out and the effect of the adhesive and plate thickness on the stress distribution at the interface was investigated. Moreover, to evaluate the fatigue strength of adhesive joints quantitatively, the stress intensity factors for interface cracks in the adhesive joints were analyzed by using the BEM. The extrapolation method proposed by the authors was used to determine the stress intensity factors. It was found that the stress intensity factor range ΔK_i=√ΔK²₁+ΔK²₂ can well characterize the fatigue strengthes of SLJ type adhesive joints with different adhesive and plate thicknesses. It is expected that the interfacial fracture mechanics is very useful for evaluating the fatigue strength of the adhesive joints.