Experimental studies have been carried out about the particle separation performance of single and two inlets hydrocyclones. Two types of hydrocyclones having the same total inlet holes area were examined. Hydrocyclone A (with single inlet hole) and hydrocyclone B (with two inlet holes) were used. An aqueous slurry of silica particles (0.5 wt% with a median diameter of about 2.1 mu m) was tested using a 20 mm-diameter hydrocyclone with a 15% underflow ratio. It was found that the optimum flow rate which gives the highest separation performance of the hydrocyclone B is the same flow rate for each inlet. Usage of the hydrocyclone B indicated smaller cut size and higher particle collection efficiency compared to that of the hydrocyclone A, under the same flow rate or same pressure drop conditions. The time of flight model was used to predict the 50% cut size diameter for single inlet hydrocyclone, and the model was modified to the two inlets hydrocyclone. It was found that the calculated results agree well with the experimental results for the hydrocyclone A, and for the hydrocyclone B under the low total flow rates conditions. (C) 2008 The Society of Powder Technology Japan. Published by Elsevier BV and The Society of Powder Technology Japan. All rights reserved.