Abstract: A stochastic-based method of evaluating the interaction parameter (F12) of the Tsai-Wu strength theory has been presented in this paper. Treating all strength parameters of the strength theory as random variables, the mean and standard deviation of F12, under plane stress conditions, have been estimated for Douglas-fir laminated veneer. This estimation has been managed through a nonlinear least-square fir of the parameters to a cumulative probability distribution of off-axis tensile data. For the purpose of comparison, a sample evaluation of F12 using deteministic methods has been presented. This evaluation showed strong dependence of F12 on angle grain. A subsequent sensitivity analysis of the off-axis tests on the value of F12 indicated that data 15° off-axis tensile tests were more stable in establishing F12 than that of other angles tested: 30°, 45°, and 60°.