Multi-layer Insulation with polyimide foam (PF-MLI) has been studied as a new insulator for next generation spacecraft. PF-MLI has higher insulation performance than that of conventional MLI as we showed in our previous work. In the case of the mission which needs high thermal insulation performance such as deep space or lunar exploration mission, it is required to cover an antenna with MLI. However, MLI does not transmit radiowave because it is composed of multiple layers of metalized films with low emittance. In this work, we have been studying a radiowave transmissive MLI based on PF-MLI by using a new thermal control material instead of metalized film. This new thermal control material is called as COSF (Controlled Optical Surface Film). COSF consists of a polyimide film for substrate and dielectric multi-layer coating on it. The solar absorptance and infrared emittance can be freely controlled by the interference of its dielectric multi-layer, and the COSF has radiowave transmissivity. The COSF is designed by genetic algorithm method. The information of multi-layers, such as materials and thickness, is encoded as a gene and evaluated by an evaluation function. The solar absorptance of COSF4 used on the outer cover of PF-MLI is 0.06 and the normal emittance and total hemispherical emittance is 0.76 and 0.70 at 300 K respectively. On the other hand, the normal emittance and total hemispherical emittance of COSF-IR2 in the middle layer of PFMLI is 0.14 and 0.17 at 300 K respectively. These thermo-optical properties has been measured by monochrometer, FTIR at room temperatures and calorimetric method in the temperature range from 173 to 373 K. The Radiowave Transmissive Multi-layer Insulation called as RT-MLI has been made by combining polyimide foam with COSF and its effective thermal conductivity has been measured by guarded hot plate method in the temperature range from 162.5 to 350 K. The density of polyimide foam is 6.67 kg/m3. The prototype of RT-MLI is composed of 2 layers of polyimide foam and 1 layer of COSF in the middle of them. As a result, the effective thermal conductivity of RTMLI is reduced by 40 % compared to that of polyimide foam and similar to that of PF-MLI. This RT-MLI will be used for a lot of mission in near future.