Thermal grease, as a thermal interface material (TIM), has been extensively applied in electronic packaging areas. Generally, thermal greases consist of highly thermally conductive solid fillers and matrix material that provides good surface wettability and compliance of the material during application. In this study, the room-temperature liquid metal (a gallium, indium and tin eutectic, also called Galinstan) was proposed as a new kind of liquid filler for making high performance TIMs with desired thermal and electrical behaviors. Through directly mixing and stirring in air, liquid metal micron-droplets were accidentally discovered capable to be homogeneously distributed and sealed in the matrix of methyl silicone oil. Along this way, four different volume ratios of the liquid metal poly (LMP) greases were fabricated. The thermal and electrical properties of the LMP greases were experimentally investigated, and the mechanisms were clarified by analyzing their surface morphologies. The experimental results indicate that the original highly electrically conductive liquid metal can be turned into a highly electrically resistive composite, by simply blending with methyl silicone oil. When the filler content comes up to 81.8 vol. %, the thermal conductivity, viscosity and volume resistivity read 5.27 W/(m · °C), 760 Pa · s and 1.07 × 107 Ω m, respectively. Furthermore, the LMP greases presented no obvious corrosion effect, compared with pure liquid metal. This study opens a new approach to flexibly modify the material behaviors of the room-temperature liquid metals. The resulted thermally conductive however highly electrically resistive LMP greases can be significant in a wide variety of electronic packaging applications.