Fused Filament Fabrication (FFF) could replace injection moulding for some applications as the shaping step in a process similar to powder injection moulding (PIM). Herein after shaping by using a highly-filled polymer the part is debound and sintered to obtain a solid part of metal or ceramic. New feedstock materials have been developed that can be printed using conventional FFF equipment, debound and sintered to obtain stainless steel parts. However, there are many parameters that can affect the performance of the FFF feedstock materials. One important parameter is the particle size distribution of filler particles. In this paper, feedstocks containing 316L steel powder with different particle size distributions were characterized in terms of viscosity and mechanical properties, and tested regarding the printability using a conventional FFF machine. It has been observed that particle size significantly affect the properties of feedstock materials.