Altitude smearing involved in MST radar measurements with low altitude resolutions is discussed, using the data obtained at Arecibo with an altitude resolution as high as 150 m. The smearing is of great concern in the case where the altitude resolution is as low as 3 km. The wind velocity deduced with the 3-km resolution is biased from the true velocity where the wind shear is large. The bias of the order of 10 m/s persists for hours. Moreover, there appear spurious velocity fluctutions, which do not correspond to any of the fluctuating components in true velocity variation. Also, most of detailed layered structures detected with 150-m altitude resolution are smeared out with 3-km resolution. The present result strongly suggests that we should be careful enough in interpreting MST radar data obtained with altitude resolutions as low as 3 km.