Hydrocyclones for separating light dispersed phase are applicable to various processes in the separation of light dispersion. The relationship of the structure parameters and flow field is in the frontier of the research and not clear so far. In this paper, Phase Doppler Particle Analyzer (PDPA) was used for the first time to systematically study the influence of the change of structure and size of vortex finder on the zero axial velocity wave zone (ZAVWZ), the tangential velocity gradient, the centrifugal separation factor and the size distribution of dispersed phase. The hydrocyclone was designed with diameter of Phi 35 mm, hollow glass beads (d(50) = 30.61 mu m, rho = 0.82 g/cm(3)) were used as the dispersed phase and water was used as the continuous phase. The test results were used as the evaluation index to optimize the depth of vortex finder at 0.143L(s) and its diameter at 0.071D. The performance of the hydrocyclone was also examined under the same material and flow condition as the PDPA test for flow field, which showed the separation performance of the optimized hydrocyclone reached up to 92.3%, while the inlet flow rate was 1.5 m(3)/h and the split ratio was 8%. At last, the reliability for taking the test results, the length of zero axial velocity wave zone (ZAVWZ), the centrifugal separation factor and the tangential velocity gradient as the evaluation index were also verified. (C) 2011 Elsevier B.V. All rights reserved.