Abstract: Surface flow visualization has been carried out with tufts adhered to the surfaces of the rotating propeller blades.From the experimental results it is found that in part of four fifths of blade surface from hub to tip,the air flow runs from leading edge to trailing edge in the outward direction with some inclination to chord.In the region of one fifth near the blade tip,flow separation and reverse flow occur at the blade surface,circulation appears even in the serious case,in consequence negative thrust emerges,which cuts down the efficiency of the propeller obviously.