The key of quantificational evaluation of petroleum resources in a basin is the division of assessment units based on the composite total petroleum system. Tarim Basin is renewedly divided into 4 composite total petroleum systems and 5 assessment units on the guidance of principle and means of the composite total petroleum system as well as the method of comparing and identifying petroleum origin. Based on detailed analysis of the prospecting and discovery history of these assessment units and integrated with discovered or proven traps, the petroleum resources, the scales and the quantity of fields in Tarim Basin are quantificationally evaluated by means of the 7th impending method and Monte Carlo method. The result of research indicates that there are 1143×106t and 210×109m3 of undiscovered potential oil and nature gas respectively, 94 oil fields, 86, of which are distributed in the assessment units of Lower Palaeozoic marine platform, and 104 gas fields, 58, of which are distributed in the assessment units of Kuqa forelandl. It is shown that nearly 91, of oil and 84, of nature gas resource are undiscovered in Tarim Basin so that it is a basin of low extent of discovered resources in land basins.