Abstract: The failure modes of two-sheet resistance spot welded joints of advanced high strength steel (AHSS) have been studied. Different failure modes of welded joints will affect the failure mechanism, mechanical properties and fracture location. Based on the experiments results of critical nugget size, peak load, macroscopic fracture morphology, failure initiation location, microstructure and micro-hardness curve in different groups, the influences of sheet strength factor and sheet thickness factor on the failure mode are illustrated. The results show that the sheet strength directly affects the failure mode, the failure initiation location and the peak load. Also the sheet thickness affects the failure mode but does not change the initial fracture location and the peak load. The factors influencing the critical nugget size (D_CR) include the sheet thickness, the sheet strength, the defects in the nugget and the distance from the fusion line at the position where the failure is pulled out. On the basis, a prediction model and method of the critical nugget size is proposed, which is in good agreement with the experimental results. And it provides a theoretical evidence for the determination of critical nugget size in practical industrial applications.