The flow rate is one of the most familiar indexes showing fluidity of powders. In commercial iron powders, it has been well known that a water atomized iron powder (WAIP) flows faster than a reduced iron powder (RIP). However, we found that the filling density of the WAIP in the narrow cavity was significantly lower as compared with that of the RIP. In this paper, we investigated the mechanisms that obstruct the die filling of the WAIP by focusing on the distinctions in the following powder characteristics: particle density, roughness on the particle surfaces, and contents of fine powder particles. The influences of these factors were examined by numerical and experimental procedures. We simulated the filling behaviors of mono-sized particles that had different particle densities based on discrete element method (DEM). The DEM simulations showed that the particle density did not have any influences. Furthermore, we examined the packing densities in the narrow die cavity for the sample powders: a WAIP having the rough particle-surfaces attached with fine iron particulates, and WAIPs sieved including various contents of fine particles smaller than 45 μm. The roughness on the particle-surfaces did not have any effect on the filling, but the reduction of fine powder content improved the incomplete filling especially in the narrow die cavity. To investigate the role of the fine powder, we measured powder yield loci (PYL) of the sample powders, and found that the share stress and the inner angle in the powder beds increased associated with the content of fine powder particles. These results indicate that the fine powder particles built the rigid powder structures that make it difficult for powder particles to be discharged from the shoe or to be packed in the die cavity. Therefore, the reduction of the content of the fine powder seems to be one of the most effective solutions to provide the homogeneous filling, and to reduce the variety in the green density of a complicated-shaped part.