This paper discusses the collapse mode of thin reinforced concrete (RC) plates subjected to blast load.To extend the well known plastic-mode method to analyze,not only perfectplastic plates,but also RC plates,it is needed to investigate the effect of material cracking on the collapse mode because the plate might have been cracked on both upper and lower surface before the plastic-mode fully develops,creating an unexpected type of collapse mode shape.A new failure mode is proposed and verified by numerical analysis in this paper.The new mode is a result of the material cracking and has an un-negligible effect on the reaction mechanism of the RC plate to the blast load.