Unseady fluid flow and heat transfer on a heated rotating disk are investigated by using a three-dimensional numerical method. It is found that the flow field changes from a forced convection flow to a mixed convection flow as the Grashof number increases. At the high Grashof numbers, vortices appear on the disk and move in the radial and azimuthal directions with stretching in the azimuthal direction. These vortices affect heat transfer on the disk. The growth mechanism of the vortices is discussed in the calculation of the production of turbulent energy. The critical Grashof number where the transition of the flow field occurs and averaged Nusselt numbers on the disk are in good agreement with available experimental data.