This paper shows a numerical research on the unsteady flow field inside a squirrel cage fan. The studied features are both the instantaneous flow fields and the average fluid flow associated to the blade passage frequency. The squirrel cage fan studied is a small centrifugal fan with a twin impeller configuration, each with 23 forward curved blades. The blades chord is 0.013 m and each impeller has a diameter of 0.08 m and a width of 0.094 m. The impellers operate inside an external spiral casing with a rectangular exit, followed by the outlet duct. A first series of experimental tests were performed in order to characterize the unit. The performance curves (head, power and efficiency versus flow rate) were measured. These tests show a nominal flow rate at around 0.098 m3/s and a specific speed ωs = 1.9. From there on, three different flow rates were considered to study different flow behaviours in the impeller. In parallel with the mentioned experimental study, the unsteady 3D flow field inside the fan equipped with the same impeller was modelled for the referred flow rates, by means of the commercial CFD code FLUENT. To facilitate the modification of any geometrical feature, the mesh of the modelled fan was divided in several regions: inlet duct, impeller, volute and diffuser with outlet duct. The main goal of the paper is to show the numerical results obtained on the absolute and relative frames. Three main flow features will be analysed: the inlet flow distribution, the blade to blade field and the impeller exit flow. At the fan inlet, special care will be taken to detect possible recirculation or separation zones. On the other hand, and for each studied flow rate, the distribution of outlet flow field is also analysed. Conclusions on flow uniformity are drawn.