Abstract: The loess is of importance to understand the progress of environmental evolution in Northwest China, which should be based on the reliable age. Because of the limit of the dating means, the conclusions on the age of loess along the north flank of the Tianshan Mountains are controversial, which makes it difficult to understand the environmental implication of the loess. Loess-paleosol series and the dating method based on the structure of geomorphologic complexs make it possible to date the geomorphologic complex and the related deposites. This study aims at giving the age of the loess along the north flank of the Tianshan Mountains. Based on the detailed field work, seven fluvial terraces have been identified along major river channels, and the distribution of loess is thought to be related with these terraces and corresponding alluvial fans. The oldest loess in the north piedmont of the Tianshan Mountains is distributed on the highest terrace T7, and the younger loess on the T6 is more extensive. Loess-paleosol series and the ESR age indicate that the age of the oldest loess in the north piedmont of the Tianshan Mountains is 0.54 Ma.