Abstract: The three dimensional elastic stress in the flat slab with crackles subjected to tension is frequently analyzed in mechanics. As entering into plastic stage, the stress status near the crack tip is different with that in elastic stage. This paper analyzes the plastic stress status near the crack tip with the FEA method for the flat slab with crackles in tension, and then discusses the influences of the thickness of the flat slab and crack-tip radius on it. The analytical results show that: (1) the decrease of crack-tip radius increases the stress and in the plane, the stress in the direction of thickness and the plane strain factor as well, (2) the plane strain factor goes to 0.5 in plastic stage, (3) the thickness has little influence on the stress and in the plane but increases the stress in the direction of thickness and the plane strain factor, (4) the plane strain factor goes to the Poisson’s ratio under small loading or to 0.5 under large loading.