Model experiment and numerical simulation are carried out in order to make clear the generating mechanism of non-uniform charging by charging roller with DC voltage. From the experiments using metal roller and polyethylene terephthalete (PET) sheet, the periodic charging patterns can be recognized on the PET sheet by cascade development with toners after the sheet is charged. The period and the size of the patterns become large as the applied voltage increases. The shapes of the patterns are different between positive and negative charging. The characteristic of the patterns is similar to that of separating discharges on an electrified insulating sheet. The results show that the charging patterns are made as follows. (1) Abnormal discharge occurs between the charging roller and the PET sheet, and large amount of charge is deposited on the sheet. (2) The charge is forced to move due to surface discharge. On the other hand, two-dimensional electrostatic simulation of the charging roller is carried out with consideration of the surface discharge as well as the abnormal discharge. From the simulation, the periodic charging patterns can be generated on the PET sheet and may verify the new model of generating non-uniform charging patterns as proposed above.