Abstract: In this paper, the triaxial inundation test of the unsaturated primary loess sampled from a pumping project along the Yellow River in Shaan Xi is described, and the analysis on the micro sturcture of the sample with the electronmicroscope before and after collapsing is done. The relation between loess structural behavior and collapsibility is confirmed through the link of micro structure and collapsibility, and the research implies that the micro structure of loess has a close relation with the structural behavior and collapsibility of loess.