Abstract: Although, many technologies are being developed for the increase of vehicle’s engine efficiency, these are based on smooth air flow for intake and exhaust. In this study, the flow situations were derived through a simulation analysis to secure basic data for the improvement of flow as a function of diameter and curvature of exhaust port. Analysis is carried out with the diameters of exhaust port into 12 and 20 mm and flow characteristics under diversified environments are evaluated as a function of height of valve lift. To simulate discharge characteristics of the exhaust gas inside combustion chamber, differences in the pressure were set up between combustion chamber interior and atmosphere and environments upon discharge of the air of a high temperature to outside were derived through a simulation analysis. When the diameter of exhaust port was increased, the pressure difference between inside and outside of the combustion chamber could be seen to be increased by 2.4% for the diameter of 20 mm compared with 12 mm while the inside flow rate could be affirmed to be higher by 0.5% for the diameter of 12 mm. However, as higher occurrence of Eddy currents for complete discharge of residual exhaust gas inside could be observed for the diameter of 20 mm, correlations in the combustion chamber as a function of diameter and curvature of the exhaust port could be identified. By providing the basic data for design of exhaust port to discharge exhaust gas based on the present study results, it is considered that a contribution can be made to optimization design.