Three–dimensional numerical simulations of an isothermal swirl fluidised bed (SFB) system with no chemical reactions were performed to study the swirl decay process and its effects on bed performance, i.e. particle concentration in the freeboard. The commercial CFD code package ANSYS 11.0 was chosen to carry out our simulation. We used 200?m glass beads with property similar to Geldart group A particles as the particulate phase with assumption that they are cohesionless and mono-dispersed, and air at 25°C as the gas phase in our simulation. Different initial swirl intensities were controlled by varying the secondary gas velocities. The solids volume fraction of solids suspension layers in the freeboard agreed generally well with literature description, which showed an annular shape with high values near the wall side. For the first time, swirl intensity and its decay process were predicted numerically by incorporating a swirl number function into ANSYS 11.0 and its effects on the bed fluid dynamics were also discussed in a qualitative or semi-quantitative manner.