Nowadays, the utilization of Oil Palm Shell (OPS) as lightweight aggregate in concrete especially in the structure application has become prevalent. As an industrial waste product, Oil Palm Shell (OPS) possibly will be the alternative material to be employed in the construction industry. With its advantage as heat resistant material, this study will focus on the potential of OPS as lightweight aggregate with regard to the optimum content of OPS for thermal insulating material. A total of 15 mixes were prepared and tested with 3 different cement/sand ratios (1.7, 1.8, 1.9) and 5 different cement contents (300, 350, 400, 450, 500 kg/m³). The result of this study show that the highest sand used will produced good workability but increased thermal conductivity of mix value. The test result indicates that the thermal conductivity and insulation criterion is substantially improved with the volume use of OPS and strong relationship between thermal conductivity and unit weight is obtained. The measured thermal conductivity value range from 0.54W/mC to 1.1 W/mC. The ideal value for semi structure insulation material establish by RILEM only achieve for mix that used cement content 400 kg/m³ and below, which thermal conductivity is 0.75 W/mC below.