Objective: To study differences between the essential oil of Thymus mongolicusfrom Gansu by GC/MS. Method: The essential oils were extracted with steam distillation method.The relative amount of each component was calculated in area normalization. The compounds in oils were isolated by column chromatography to identify the chemical constituents ofessential oils as assistant method. Result: Eighty-nine compounds were identified in the three essential oils. The major constituents of oils from Dazonghu region were geraniol

thymol and longifolene

and from Zhayougou were linalool and thymol

ect. The major constituents of oils from Tianshui were sabinene

eucalyptol

thymol

cedrol

ect. Conclusion: The main chemical component of essential oil of T. mongolicus in Gansu were monoterpenes. The differences in growing environment were different with relative content of essential oil composition.