This article is an experimental program that was undertaken to investigate miscible gas (methane) injection effect on oil composition during enhanced oil recovery process. In order to perform the injection operation and calculating minimum miscibility pressure, a slim-tube apparatus was used. After that, geochemical tests were employed to investigate the effect of methane injection on oil composition. A chosen oil sample (API 30.5) was taken from "B" field on the eastern side of the Persian Gulf. Although polar components have been affected by the methane injection in a way that asphaltenes and resins content were decreased after injection, non-polar compounds have not been changed at all.