Abstract: This paper explores the fully plastic J integral expression for two-dimensional flat plate model of Nozzle Corner Cracks. A new expression is giver on the basis of the shape coefficient of stress intensity, factor β. Through discussion of the plastic factor ω in the expression and the mathematical analogy between two-dimensional and quasi-three-dimensional models, a full, plastic solution of J-integral for the quasi-three dimensional model is obtained which is dependent upon existing two-dimensional model. It is shown that the bearing capacity will be 10% - 20% higher than that of the two-dimensional model when the three-dimensional model is employed Therefore, it is concluded that the three-dimensional model is less conservative than the two-dimensional plate model.