A constitutive model for characterizing high strain rate response of polymeric composites was developed using off-axis composite specimens. The difference in the results of coupon and block specimens was experimentally demonstrated and discussed. Off-axis block specimens were tested in compression at three different strain rates, 10-4/s, 10-2/s, and 1/s. Based on these experimental data, a viscoplasticity model was established to describe the nonlinear rate dependent behavior. This viscoplasticity model, although developed based on data for strain rates up to 1/s, was verified with data obtained from high strain rate experiments conducted on a Split Hopkinson Pressure Bar (SHPB) using block off-axis specimens. The axial strain rate in the SHPB tests were in the range of 400-700/s. Comparison of model predictions with experimental data indicated that the viscoplasticity model determined using low strain rate tests was valid for high strain rates at least up to 700/s.