Finite element method is extensively used for the analysis of impact response in complex materials. The prediction from finite element model may exhibit significant difference from that of experiments due to uncertainties in model, experimental measurements, and parameters that are derived based on experiments for model development. The quantification of parametric uncertainties, such as parameters in constitutive relation, associated with the numerical model is an important aspect that needs to be investigated for a credible computational prediction. This work considers uncertainty quantification in finite element modeling of nanoscale dynamic impact problems. A viscoplastic power law constitutive model is obtained from nanoscale impact experiments on Hydroxyl-terminated polybutadiene (HTPB)-Ammonium Perchlorate (AP) samples. The constitutive model is used in a finite element model to simulate impact experiments. The measured response from impact experiment and FEM simulation is used to quantify the parametric uncertainties in the constitutive model for the analyzed HTPB-AP sample. © The Society for Experimental Mechanics, Inc. 2019.