The North Tianshan belt was formed during closure of the North Tianshan Ocean. Detailed study on the geological unit within the North Tianshan belt is crucial to reveal tectonic process of the North Tianshan Ocean. There are several granitic plutons intruding into the North Tianshan belt, but detailed studies including geochemistry and geochronology are limited. In this paper, we conducted zircon U- Pb dating, geochemistry and zircon Lu- Hf isotope analyses on the Saersala and the Ailuxun plutons in the western part of the North Tianshan belt. These data were used to discuss the petrogenesis and geological setting of the plutons. The Saersala and the Ailuxun plutons are both monzogranites. Their zircon U- Pb ages are 303 Ma and 298 Ma. Zircons from the two plutons all display high positive εHf(t) values, which are 9. 7 to 12. 1 (average 11. 2) for the Saersala pluton and 12. 1 to 15. 0 (average 13. 2) for the Ailuxun pluton. Their two stage model ages are 696~547 Ma and 541~357 Ma, respectively, reflecting juvenile magma sources for the studied plutons. Samples from the two plutons all have high SiO2 and total alkaline contents, and belong to weakly peraluminous high- K calc- alkaline granites. They have high CaO/Na2O values and Al2O3/(MgO+FeOT)mol values, indicating magma sources mainly of meta- greywackes. The flysch sandstones in the North Tianshan belt are potential source rocks for the studied monzogranites. Combine with other geological evidences, we suggest that the Saersala and the Ailuxun plutons were formed during the transition stage from collision to post- collision. The North Tianshan Ocean closed during the end of the Late Carboniferous.