Excitations by various actuators on satellites often degrades performance of observation devices equipped on the satellites, even though these are very small. Reaction wheels are considered most responsible among various types of vibration sources. In this paper, passive vibration isolations for a reaction wheel assembly (RWA) are presented. An engineering model of the reaction wheel was manufactured and its dynamic characteristics were identified by measuring blocked forces and modal test. The reaction wheel and a rigid mass in place of an observation device were installed on a cantilever beam for simplicity, which can be regarded as vibration path in real satellites. Isolators were designed by taking into account characteristics of the reaction wheel as the excitation source, structural paths between the source and the receiver, and angular vibrations at location of the receiver. For the purpose of comparison, another set of isolators were designed simply from the point of conventional transmissibility under the assumption that the receiver structure is rigid.