The Effects of off-bottom clearance on Macro instabilities (low frequency fluctuations of flow) generated by a Rushton turbine in a fully baffled cylindrical flat-bottomed vessel was investigated by laser Doppler velocimetry and a laser sheet flow visualization. The frequency of Macro instabilities depended on off-bottom clearance C, that is, the volume between the impeller and the bottom or the surface. Two frequencies of Macro instabilities above and below the impeller in the range of 0.3D less than or equal to C less than or equal to 0.8D showed linear correlation to the off-bottom clearance C, where D is the vessel diameter. However, for C < 0.3D or C > 0.8D, the frequencies did not follow the linear correlation to the off-bottom clearance C.