This paper presents a numerical study of particle-fluid flow in complex three-dimensional (3D) systems by means of Combined Continuum and Discrete Method (CCDM). In the CCDM, the motion of discrete particles phase is obtained by Discrete Element Method (DEM) which applies Newton's laws of motion to every particle and the flow of continuum fluid is described by the local averaged Navier-Stokes equations that can be solved by the traditional Computational Fluid Dynatnics (CFD). This method has been increasingly used worldwide, but so far its application is limited to relatively simple flow systems. In this work, the simulation is achieved by incorporating a DEM code into the commercial CFD software package Fluent that can be readily used for complex CFD problems. The applicability of this development is demonstrated in the study of the particle-fluid flow in various 3D systems including pneumatic conveying bend, cyclone separator and circulating fluidized bed. It is shown that the numerical results are, either qualitatively or quantitatively depending on the availability of experimental data for comparison, in good agreement with those measured, and can generate information leading to better understanding of the internal flow structure of these systems. (c) 2007 Elsevier BY All rights reserved.