Linear viscoelastic region is important to investigate material properties. We investigated boundary of linear viscoelasticity for simple shear flows with various time functions of shear strain. We used Phan-Thien and Tanner (PTT) model and Giesekus model in order to extract the linearity criterion. We determined critical strain (γc) as a function of dimensionless number such as De or Wi and the nonlinear parameter of the models. We found a superposition of the critical strain at start-up shear flow and oscillatory shear flow. Replacing relaxation time by mean relaxation time (λ=Jeηo , we checked the validity of the equation with experimental data.