Abstract: In order to study theeffect of grain size on separation performance of dense medium cyclone,the particle size and density distributions of feed and products of cyclone were carried out,and then the probable error and misplaced material content of each size fraction were calculated and comparative analyzed.The results show that each size fraction had different separation performance.The size fraction of 13~6mm was got the best separation performance,and the larger size and the smaller size were got the worse separation.The size fraction of 13~6mm was got the best separation performance,the values of probable error andtotal misplaced material were 0.043 and 1.6%,respectively.At the grain size greater than 13mm,the separationperformance became poorer,and the values of probable error became larger,and total misplaced material was increased to 1.7%.At the grain size less than 6mm,the separationperformance became poorer obviously,and the values of probable error was greater than 0.050,and total misplaced material was increased to more than 2.0%.The size fraction of 13~6mm was got the best separation performance,and the larger size and the smaller size were got the worse separation.