Accounting for the nonlinear stress-strain behavior of plastics is important for good design of a snap joint. A constitutive equation for approximating the nonlinear stress-strain behavior of plastics is introduced in this paper. The equations for the stress, strain, and deflection of a cantilever beam, based upon the newly introduced constitutive equation are presented. In comparison with the experimental results, the cantilever deflection predicted from the nonlinear analysis is found to be about 50% more accurate than the corresponding predictions from the linear analysis.