Small diameter (10 mm) hydrocyclone is used for catalyst particles separations in this work. An industrial sidetrack tester was set up in 1.8 Mt/a catalytic cracking unit. When Reynolds number is invariable, the experimental results show that the pressure drop between the inlet and the overflow is not changed by changes in the split ratio. The effects of Reynolds number and split ratio on separation efficiency were studied. The removed efficiency of catalyst particle is more than 55% with Reynolds number of 850 similar to 950.