The magnitude of constraint effect $A_2$ values on the non-linear elastic plastic fracture toughness was experimentally estimated by using displacement at various measuring positions near crack tip. Constraint effect $A_2$ value was dependent on specimen configuration and on the measured displacement near crack front. The crack tip opening displacement in the vicinity of the crack tip front should be estimated within plastic region when appropriately constraint effect was calculated. It was found that the magnitude of constrain effect |$A_2$| is below 8.0 at the crack tip. But an appropriate location to measure the effective constraint effects $A_2$ at the critical value of J that crack initiation is characterizable by is r = 2mm and ${ heta}=90^{circ}$ away from original crack tip, and the constraint effect |$A_2$| estimated is 5.3.