Boron nitride (BN) has extremely high thermal conductivity among ceramic materials and electrically insulating property. In this work, the thermal conductivity of boron nitride, alumina and graphite filled polydimethylsiloxane (PDMS) was studied. Three different particle sizes of boron nitride which is 5 ƒÝm, 10 ƒÝm, 20 ƒÝm were used. Generally, the particles size of the filled-materials in polymer matrix plays an important role in light of improving thermal conductivity of their composite. PDMS composite was fabricated using two-roll mill which made it possible to simultaneously control mixing, hot-press molding for polymer curing, and air-press. The thermal conductivity of these composite is shown to increase in accordance with the concentration of filler. As mentioned above, there is a tendency that the larger size of boron nitride particle, the higher thermal conductivity of composite. PDMS/BN, PDMS/alumina composites have an electrically insulating surface unlike PDMS/graphite. The resulting composite was characterized by Quick Thermal Conductivity Meter (QTM-500), Scanning Electron Microscope (SEM) and electrical conductivity measurements. The mechanical properties including tensile strength, curing, and hardness were analyzed by Universal testing machine (UTM) and shore-A, Rubber Process Analyzer (RPA-2000). Keywords: Poly(dimethyl siloxane), Boron nitride, Composite, thermal conductivity. Acknowledgement This study was supported by Regional Innovation Center for Environmental Technology of Thermal Plasma (ETTP) at INHA University designated by MOCIE (2014)