The IsaMill (TM) is a high speed stirred mill with a horizontal configuration that offers advantages such as energy efficiency and an inert grinding environment. A combined computational fluid dynamics (CFD) and discrete element method (DEM) approach was developed to investigate the particle and fluid flows inside a simplified IsaMill (TM). The configuration of the mill was simpler than that of an actual IsaMill (TM) and no feed flow or rotor was considered. The CFD-DEM model is a progression from earlier DEM only models of dry systems which did not account for the fluid phase. The properties of flows at a macroscopically steady state, such as velocity field, distributions of particle velocity and acceleration in the radial direction and power draw, were analysed. Detailed comparisons were carried out between the simulation results and Positron Emission Particle Tracking (PEPT) measurements under similar conditions. The comparisons showed reasonable agreements, confirming that both techniques can capture the key features of the flow. The discrepancies between simulated and measured results were discussed. The findings indicated that the proposed model can be used to generate microdynamic information that is useful in leading to a better understanding of the underpinning physics of flow inside mills.