This paper points out that there are different concepts and division standards for low-maturity oil and its source rocks. The author discusses the division of low-maturity oil and its rocks, and attempts to define the low-maturity oil. In addition, hydrocarbon-generating material of the oil is reviewed and its definition is given. On that basis, the author points out that material of the oil generation has the multi-component characteristics and that the oils have the multi-source characteristics, and, further, it is indicated that oil formation is a process of multi-source and multi-component compounding and that the process is a part of the continuous hydrocarbon generation model of the damped cosine curve during the burial of sedimentary organic matter in geological time. All the above-mentioned ideas are very important for a correct evaluation of low-maturity oil-gas resources in petroliferous basins.