Because of undesirable effects of cavitation, the design of a cavitation free structure is an important problem in the field of Hydraulic Engineering. The surface of an ordinary hydraulic structure has roughness, scattered randomly or isolated. Only few investigations have been done on the effects of surface roughness on cavitation. Therefore, “the effects of randomly distributed surface roughness on cavitation” is investigated in this paper. For the experimental investigation of the problem, a circulating type 6 inch water tunnel is designed and constructed at the Hydraulic Laboratory of Civil Engineering Department, the Pennsylvania State University, U.S.A. The test section of the tunnel has a cylindrical cross section with an inside diameter of 6 inches and a length of 24 inches. An axial flow propeller pump (9 inches in diameter), driven by a two speed motor, is installed for the circulation of water. The velocities in the test section corresponding to high and low motor speed are 43.52 and 32.42 feet per second. The static pressure in the test section is controlled by the pressure control system and a vacuum pump. The range of the static pressure is from 2 psia to 85 psia. The models employed in the test are three families of 3/4 inch diameter stainless steel noses having 1.5 caliver ogive, hemispherical and blunt profiles. Each family consists of four noses having different surface roughness. The first one is polished with hand, the second is roughened with sand blasting and the third and forth are roughened with steel shot. The surface roughness ranges from 2.8 to 580 p in rms. The test is conducted in the 6 inch water tunnel, and consists of finding the desinent cavitation number and the incipient cavitation number under different conditions. The air content of water is measured for each test run using a Van Slyke equipment. The test results show that, in general, the cavitation number for the onset condition increases significantly as the surface roughness increases. In addition, it is seen from the results that there is a close correlation between the air content of water and the onset of cavitation. In general, the cavitation number for the onset condition increases as the air content of water increases.