From the probable viewpoint that the collection characteristics will depend upon the design of the unit and the operating conditions, performance tests were carried out using an experimental unit at Resources Research Institute, Agency of Industrial Science and Technology. This paper gives the results of the work, including some suggestions with respect to the collection efficiency. The influences of the following factors upon the collection efficiency ηe were investigated : (1) Feed inlet velocity νi, (2) Diameter of overflow finder Do, underflow nozzle Du and cylindrical portion D, (3) Dimension of inlet nozzle √(bh), (4) Effective height of cyclone He, (5) Inserted length of overflow finder S, (6) Types of feeding, (7) Pulp density δf and particle size d of feed slurries. In addition to the above results, the author has shown the following two theoretical formulas : (1) Critical feed inlet velocity corresponding to the maximum collection efficiency ηe(max) may be expressed as Eq (4·4), (2) Collection efficiency ηe may be expressed as Eq. (4·7).