In this work, yielding and post yielding effects are analyzed using the idea of the inhomogeneous distribution of strain in polymers under deformation. This idea is directly connected with the free volume concept, as created in the polymeric bulk during the frozen in process, and or the density fluctuations combined with other type defects. A simple strain density distribution function is assumed, following experimental results available from techniques such as positron annihilation lifetime spectroscopy (PALS). Hereafter, a functional form of the rate of plastic deformation is extracted, which will be combined with a given kinematic formulation. The proposed analysis is tested with experimental data of polystyrene (PS) and PS/SiO2 nanocomposites. The incorporation of nanosized fillers into the polymeric bulk, strongly affects the free volume distribution and/or the distribution of defects related with density fluctuations. This fact is reflected in the model parameter values, and their variation in respect to the different material types. With the proposed analysis, it has been proved that all features of yield process, including strain softening and strain hardening effect are easily described. (c) 2008 Elsevier Ltd. All rights reserved.