Abstract: In this paper, attention is focused on the mechanisms of shaft frictions of vertically loaded single pile under three loading conditions:compressive load at the top of pile, tensile load atthe top of pile and uplifting load at the tip of pile. Finite element analysis is conducted to study the mechanisms. It is shown that the development and distributions of shall frictions under the three loading conditions are different. In addition, it is found that the main potential mechanisms responsible for the differences lie in the differences in stress paths in soil, rotation of the principal stress directions, the compression or extension of pile shaft and the expansion and contraction of pile shaft.