Measurements of the mean flow velocities over a disk rotating in a quiescent air are presented in this work. The measurements were performed on flat and rough disk surfaces such that the effect'of surface roughness can be investigated. The surface roughnessemployed in the present work was 0.03 mm. Measurements were extended from the laminar region to the turbulent region where the maximum value of k number employed in the present investigation is 3.7.10. A hot wire anemometer with a single sensing wire was used in the measurements. The three components of the mean velocity were separated by performing measurements in two distinct directions of the sensing wire. The transition region was detected by employing an oscilloscope jouned to the anemometer circuit. The results indicate that th_, poundary layer velocity is composed mainly of radial and tangential components in the laminar region. In the transition and turbulent regions, away from the disk surface by a small distance, the magnitudes of the three components of velocity are cf the same order. The effect of surface roughness is to cause the transition region to start at smaller values of Reynolds number and to increase the magnitudes of the three components of velocity in the transition and turbulent regions. Comparisons between this work and previous works are presented and discussed.