Micro powder injection molding (µPIM) is a preferred technology for the production of micro parts or micro structured parts which derived from the well known thermoplastic injection molding technique. It is suitable for a large-scale production of ceramic and metallic parts without ﬁnal machining. In the hardmetal industry, submicron and ultraﬁne hardmetals are the most demanding and also the fastest growing grades in production and application. Four stages involve in µPIM are mixing, injection, debinding and sintering. The volumetric ratio of solid powder to the total volume of powder and binder, which is usually called powder loading, largely determines the success or failure of subsequent processes. Critical solid loading of the powder can be estimated by torque variation, density, melt flow, density and viscosity versus composition. In this paper, critical solid loading of WC-10%Co is determined using torque variation method and its rheological behavior is studied. During the process, the wet surface of the powder particle WC-10%Co will cohesive together and resulted to the torque. Progressive powder is added-in after torque decrease and critical solid loading is identified when torque becomes unstable. Hence, critical solid loading WC-10%Co with WC (APS < 1 µm) is 46% and 42, 43 and 44 vol% of powder loading are selected to mix with wax-based binder system. The viscosity of feedstock show the pseudoplastic behavior and flow index (n) are 0.444, 0.491 and 0.492 for powder loading 42%, 43% and 44% respectively.