In this paper, the current-voltage characteristics of a graphene-based transistor in the presence of external strain are computed with different methods. A simplified semi-analytic method allows fast computation of the electric parameters by granting a good accuracy. A more complex matrix method based on second quantized version of Schrodinger equation by means of non-equilibrium Green's function is used to validate the semi-analytic one. Both models are developed to evaluate the impact of mechanical stresses on this novel class of nano-transistors, where they are aimed for applications in the domain of flexible electronics.