Abstract: Experiments were conducted to study the heat transfer characteristics of a rotating ribbed square serpentine passage for unsteady cases. Abundant experimental data were obtained to investigate the variation of heat transfer coefficient at different engine working conditions, such as mass flux and rotational speed. For unsteady cases, the convective heat transfer was affected by the cross-stream secondary flow caused by unbalance distribution of the Corilis, the buoyancy and the angular acceleration forces. Effects of angular acceleration force increase heat transfer coefficients on leading surface and decrease the coefficients on trailing surface in radial outward passage flow for the accelerating rotation system; Reverse conclusions can be obtained for the case of decelerated rotation system. It is shown that the fluctuations of heat transfer coefficients occur mainly at the moment when the work conditions vary under unsteady cases. The fluctuations of heat transfer coefficients mainly occur at the moment when the work conditions change, though long exists of the walls temperatures variation. An interesting phenomenon was also observed that there exists a hysteresis of Nu number ratio when the variation of rotational speed completes a cycle.