The simulation of print quality (PQ) defects is a process in which an image is degraded by artificially generated structures to create an impression of real defects when the image is viewed or printed. The simulation algorithms generate a set of defective images that can be used in diagnostic tools for printer users. These algorithms can also provide experimental images for psychophysical evaluations regarding defect perception, which requires the capability to control the level of the defects. In addition, simulation of defects aids in testing the usability of a PQ assessment page, and the development of computer algorithms for detecting PQ defects. Thus there are many applications that need high quality simulation of PQ defects. However, little research has been devoted to this work. In this paper, we characterize the common PQ defects in color laser printers such as banding, streaks, ghosting, and repetitive artifacts. We classify these defects into three categories: defects of uniformity, defects of random marks or repetitive artifacts, and color defects. We develop a simulation model for each defect based on its features such as the shape and the spatial distribution of the defect marks, or the tone dependency of the defect. The defects are simulated on RGB color continuous-tone images.