Recently, boost pressure and EGR ratio are set to increase for improving performance of diesel engine, and pressure in a cylinder is higher than that of conventional engine. Therefore, it is necessary to understand effect of high ambient pressure in a cylinder on diesel spray. In this study, diesel spray was formed in high pressure vessel that simulated high boost engine. Shadow images of spray were captured for understanding structure of spray, and time-resolved particle image velocimetry (PIV) was applied for observation of spray behavior. As the results, axial velocity of diesel spray was reduced with increasing of ambient pressure. Downstream spray was caught up and overtaken with upstream spray, was clarified.