Metal injection molding (MIM) process offers net shape production and high cost efficiency in order to produce the materials with poor workability such as Ti-6Al-4V. In our previous studies, injection molded Ti-6Al-4V showed high tensile strength and elongation comparable to those of wrought materials. However, fatigue strength of injection molded Ti-6Al-4V was lower than that of wrought materials because of large grain size and retained pores. In this study, the grain size of MIM Ti-6Al-4V compacts was decreased by addition of TiB2 powder and using the fine powder. As a result, the 0.4 mass% Boron added compacts treated HIP showed the fine grain size. Therefore, 93 % of fatigue strength compared to that of wrought material was obtained. In addition, the relationship of grain size and fatigue strength showed good agreement with Hall-Petch relations.