Abstract: Most of deep source rocks were in high-mature or over-mature stages,and deep oil was found in only three wells in the North Songliao Basin.Carbon isotope of kerogen was heavier in deep formations of the Songliao Basin,but lighter carbon isotope may generate because the enrichment of ¹²C in fatty side-chains.Carbon isotope was bascally not related to its thermal maturity,but was affected by its origin.Carbon isotope fractionation of oil in the deep was resulted from the primary migration of oil and the crac-king of large molecules in reservoirs.The fractionation resulted in different slopes between the n-alkane carbon isotopic profile of oil and that of its source rocks.Comprehensive analysis indicated that the high mature oil in the deep was generated by the source rocks deeper than the No.1 member of the Qingshankou Formation.