In a tandem printer, local dot positioning errors at a first transfer, which likely occur in thick paper printing, are unacceptable problems.In this study, the mechanism of generating local dot positioning errors is investigated by FEM analysis. At first, vibration characteristics of a transfer belt drive system are investigated experimentally. And then, motion and deformation of a belt, a paper and rollers with the vibration characteristics are analyzed by FEM. Finally, calculated belt velocity fluctuation is validated by comparison with experiments.As a result, it is clarified that the local dot positioning errors by belt velocity fluctuation at the first transfer are generated by belt tension and torque fluctuations on the drive shaft caused by impacts when leading edge of the thick paper is fed into a second transfer nip region. It has been known that the impacts generate a torque fluctuation on the drive shaft in a belt drive system as well as a steel rolling mill system. However, it is found in the result that they also generate a belt tension fluctuation in a pre-nip region caused by a contact force between a belt and a paper.Present calculations can also provide image quality deterioration levels affected by jitter since the practical velocity fluctuations are accurately simulated.