The isothermal problem of a mode I fracture of a shape memory alloy (SMA) plate is considered. Although the plate material is in the austenitic phase at a reference temperature, the martensitic transformation is triggered by high effective stresses at the tip of the crack. The phase transformation is accompanied by a reduction of the stiffness and strength that affects the extent of the plastic zone. The analysis is based on a two-dimensional version of the Tanaka constitute theory for SMA. The goal is to evaluate the size of the plastic zone in front of the crack and the effect of the superelastic phase transformation on the stress intensity factor. This estimate can be used for better understanding of damage tolerance of SMA plates.