Recently, industrial technology for both improving thermal conductivity and controlling the coefficient of thermal expansion of heat sink materials has became an important issuebecause of the downsizing of electronic devices. We have been investigating the innovative processing method for TiB2 dispersed Cu matrix composite by reactive infiltration process in which the combustion reaction of elemental powders (Ti+2B+Cu → TiB2+Cu) and pressureless infiltration of molten Cu into porous reaction product (TiB2/Cu composite) are combined. By this process, fine TiB2particles (2~3µm) can be dispersed in Cu matrix homogeneously. However, for better thermal conductivity and reduced thermal expansion, 3-dimentionally continuous inter-penetrating structure of TiB2 and Cu phases is suitable. In this study, we researched the effects of Cu powder size and volume fraction in Ti,B,Cu green powder compact on the microstructure of the combustion synthesized TiB2/Cu composite. When Cu powders were smaller than 45µm, TiB2 particles were uniformly dispersed in Cu matrix. However, when Cu powders were larger than 150µｍ, monolithic Cu area without TiB2 dispersion was formed. The monolithic Cu area tended to be connected each other by increasing the amount of Cu powders. This resulted in the formation of 3-dimensionally continuous inter-penetrating TiB2/Cu microstructure.