Sitting posture is one of the most important indexes for heavy truck driving comfort. It is closely related to the driver’s muscles, joint angles and joint torques. Driver’s fatigue during long hour driving comes from various causes, such factors include seat dimensions, features, adjustability and cushion’s material properties. likewise, posture changes in the performance of the seat system can contribute to changes in the long term feel of the seat and fatigue. Stability of sitting posture is characteristic to maintain initial sitting posture while driving long hour. And if the posture is likely to be changed, such as moving forward of hip-point and curvature of the lumbar, it can be called that stability of sitting posture is lack. Especially the driver of heavy truck must sit in an upright posture, because back angle(torso angle) is small(12~14˚), body pressure of driver is concentrated on the cushion and have a weak support at back pan. so, it raises problems about stability and fatigue. The purpose of this study is to clarify the mechanisms of driver’s posture in relation to seat performance in heavy truck and to improve of sitting posture stability through seat parameters change. And the object is to develop predictive and reliable CAE tools(PAM-COMFORT) and methods dedicated to sitting posture comfort assessment. This CAE methodology must be easily applicable in serial development so as to efficiently support design and package teams. For now, the focus is set on the verification of discomfort assessment via a CAE analysis using a human model and hip point machine(HPM, SAE J4002).