The experiment which temperature and pressure were near the condition ofnatural reservoir was carried out to synthesize hydrocarbon inclusions in carbonate within pure oil. Through the microscopic and fluorescent characters of hydrocarbon inclusions in carbonate, it demonstrated that carbonate remained growing and trapping inclusions when fluid medium was pure oil. The water was not necessity for trapping hydrocarbon inclusions in growing carbonate. Combined with conclusions of predecessors and wettability determination of carbonate, the trapping mechanism of hydrocarbon inclusions in carbonate was concluded as follows:coexistence of oil and water (water wetted), coexistence of oil and water (oil wetted) and pure oil.