A Hybrid electric vehicle saves fuel by four aspects, i.e. engine idle-stop, regenerative braking, engine downsizing, and engine load change. Saving fuel by 5-10% is achievable with optimized fuel cut-off strategy. However, emission and drivability trade-off must be carefully treated.The conventional automatic transmission requires a mechanical pump driven by the engine crankshaft offers hydraulic function, lubrication, and cooling. To stop the engine during vehicle idling, transmission fluid pressure will not be sufficiently maintained for the launch clutch in engaged status. Once the engine restarts, the engine speed profile can cause the transmission fluid pressure uncontrollable, which creates bump during vehicle take-off. In most vehicles equipped with a conventional automatic transmission (AT) as well as start-stop function, an electric oil pump is usually installed to maintain fluid pressure. However, cost and complexity increase, and the electric pump is redundant to the mechanical pump during normal driving.This paper presents a method for smoothing the change of automatic transmission fluid (ATF) pressure by utilizing the belt-driven starter-generator (BSG) using field oriented control (FOC) method and the correlation curve between AT fluid pressure and rotational speed of BSG. The proposed system reduced the numbers of changes and incremental cost due to additional start/stop technology applied to the existing vehicle design.