Abstract: [Objective] To investigate whether there are vertic soils existing in the northern slope of Tianshan Mountain and explore the internal mineralogical mechanism of vertic soils in Xinjiang by measuring clay mineral composition.[Method]Eleven soil profiles in the northern slope of Tianshan Mountain were studied using traditional field survey approaches.Development degree of soil vertic characteristics was determined according to diagnostic criteria such as cracks,slickenside and wedge-shaped soil aggregates,coefficient of linear extensibility(COLE)and smectite content in clay mineral of 28 soil layers were measured,and their correlation was discussed.[Result]Among the eleven studied soil profiles,four could be relatively classified as soils with strongly developed vertic characteristics.The mean smectite content of the 28 soil layers was 38;.Smectite contents of soils with strongly developed vertic characteristics were significantly higher than those of soils with slightly developed vertic characteristics.A significant power relationship existed between smectite content and COLE.[Conclusion]Low smectite content is the internal mechanism that determines the low development degree of soil vertic characteristics in this study.Distribution area of parent material with high smectite content should be especially considered when surveying vertisols in Xinjiang in the future.