Rate-dependent large deformation behavior of the alloy of polycarbonate and acrylonitrile-butadiene-styrene (PC/ABS) is experimentally investigated over a crosshead speed range of 1-3000 mm/min. Three-dimensional non-contact digital image correlation (DIC) method is used to measure the large deformation of polymer specimens. Numerical simulation of geometry effect on the necking process of specimens is done for the specimen with two section sizes. It is found that the width contracts less than the thickness due to its larger size than the thickness for specimens with rectangular sections, and the relations between two lateral contraction ratios and engineering strains are geometry dependent, but independent of loading speeds. The influence of strain rates on local volume ratios of PC/ABS is also discussed. Based on the experimental results, a simple phenomenological constitutive model with six parameters is proposed for the glassy polymer, in which the effect of strain rate and its variation during constant crosshead speed loading tests is considered, and can be used in constant true strain rate or constant principal stretch rate loading condition. (C) 2008 Elsevier Ltd. All rights reserved.