A new type of hydrocyclone with a perforated inner cylinder is tested experimentally to study its performance for liquid-solid separation. The size of the cyclone is small so that it may be effective for small particle separation. Two types of the inner cylinder were made : the one has 39.5% void fraction of the perforated surface and the other has 73.5% . The mean particle diameter is 4.5 μ m. It is found that the pressure loss of the cyclone with the inner cylinder is reduced by about 25% compared with that of the ordinary type. The new type cyclone has better separation efficiency.