Abstract: The northern Daqingshan orogen in Hohhot area is tectonically characterized by a northwest-directed thrust-nappe system, the Daqingshan thrust-nappe system, which consists of three east-northeast trending thrusts, three nappes and an autochthon. Higher-grade mylonite was formed at the bases of the hanging walls of the thrusts, while lower-grade phyllonite was produced on the top of the footwalls. Based on structural geological investigation and integrating macro-/micro-analyses of petrology and mineralogy, this study adopts 40Ar-39Ar dating method to constrain the deformation time of the Daqingshan thrust-nappe system. Three sericite samples of phyllonite give a 40Ar-39Ar age range of 120~119Ma and the other sample presents a peak age of 120Ma in the age-probability spectrum. Phyllonite was formed by a deformation at low temperatures and the fine-graded sericite in it was new-born syn-deformational mineral, therefore the sericite 40Ar-39Ar age can represent the deformation time very well. A weak-deformed granodiorite dike intruding the thrust indicates an emplacement in a late deformational stage, and its hornblende 40Ar-39Ar dating presents a lower-limitation age of 121Ma for the cooling of and the statistical peak age is also 119Ma. The lower-temperature phyllonite should be formed in a late stage of the deformation because higher-temperature mylonite developed at the bases of the nappes. Therefore, the thrusting of the Daqingshan thrust-nappe system must have been in its late deformational stage during the time form 120 to 119Ma.