ABSTRACT Heat in the room is caused by the human body, the equipment inside the room, and heat flow from outside the room. Thermal insulation material is important for decreasing excess heat entering from the environment outside the system. Thus, efforts to utilize rice straw waste to manufacture particleboard as an alternative thermal insulator are very potential. In this study, particleboards are made using a mixture of rice straw and epoxy resin adhesive. The purpose of this study is to determine the effect of particle size and composition of rice straw on thermal conductivity, density, and porosity of particle boards to obtain the best particleboard information for thermal insulators. The study purpose was carried out by varying the particle size of rice straw (20, 40, 50, and 60 mesh) by rice straw composition using epoxy resin with the comparison (vol.%) 80:20, 85:15, and 90:10 to determine its effect on thermal conductivity, density, and porosity. Particleboard manufacturing process is done by press using a hydraulic press machine with a load of 9 tons for 30 minutes in a 15cm x 15cm x 1cm mold, after mixing and stirring on rice straw particles and epoxy resin. The test results obtained indicate that the increase in the thermal conductivity of the particle board is in line with the decrease in particle size and composition of rice straw with the lowest value of thermal conductivity 0,0336 W/m o C which obtained particle board with particle size of 20 mesh and rice straw composition 90%. Density and porosity of particle boards have the opposite relationship, where the increase in density generates a decrease in porosity of the particleboard. Based on the thermal conductivity values obtained, rice straw particle board with epoxy resin adhesive has low thermal conductivity compared to thermal insulator on the market, so that it can be applied as a thermal insulator.. Keywords: thermal insulator, particleboard, rice straw, epoxy resin, thermal conductivity.