As a good shift quality and a low fuel consumption are increasingly demanded in a passenger automotive automatic transmission (in short: A/T), various new performance studies of automotive automatic transmissions have lately been introduced. In this paper, we considered the dynamic characteristics of pressure observed in hydraulic elements of automatic transmission, such as an oil pump, a pressure control valve, and a torque converter. The suppression of discharge pressure ripple using a notch is first examined in a trochoidal gear type oil pump. The improvement of hydraulic stability of clutch control pressure is also investigated on the basis of the experimental and simulation results. And, the CFD analysis shows that the key factors affecting performance of the torque converter are angles and shape of blades modified to reduce pressure loss. From these results, we may conclude that the characteristics of hydraulic pressure are primarily correlated with the performance of automatic transmission.