In previous works, the processing of plasticized PVC reinforced by cellulose nanocrystalline whiskers has been presented, as well as its mechanical behavior in the linear viscoelastic domain. The purpose of this work is now to evaluate and model the plastic behavior of such material. Compression tests are performed in the glassy state. The "quasi-point defect" or "qpd" theory, developed by Perez et al. and previously used for the analysis of the composite mechanical behavior in the linear domain, is now applied to the nonlinear domain to account for the matrix behavior. Then, an incremental approach, using the relationship between the matrix and the composite modulus deduced from DMA experiments, is used to predict the composite behavior by calculating the instantaneous modulus of this composite along its load path. Moreover, a simple model is introduced to account for the stress concentration created by the whiskers in the matrix. It explains the disappearance of the stress peak on the stress-strain curve at increasing whisker contents. Finally, the damage phenomena are discussed, highlighted by successive compression tests.