YamSat is an experimental picosatellite that is being developed by NSPO. Because it is required to weigh less than 1-kg and operate on minimal power, the passive thermal control method has been applied to YamSat's thermal design. In this paper, the concept and approaches for thermal control design, as applied to the picosatellite, will be briefly discussed. In order to fulfill the thermal control subsystem requirements the highly reliable design in thermal hardware with surface properties is adopted to meet the purpose, and the optimum design of isolators is verifed by thermal analysis. The final results show that, for each possible beta angle, the present thermal design is able to maintain each component's temperature within their allowable ranges with an uncertainty margin of 5℃.