A novel method to obtain a compact plastic package with higher isolation by providing subsidiary inner ground leads between outer leads is proposed and demonstrated. The effect of the subsidiary ground leads is investigated by using a 3-dimensional electromagnetic field simulation and measuring the fabricated packages. Newly designed package with subsidiary ground leads achieves higher isolation by more than 10 dB at 3 GHz as compared to a conventional package. This package is applied to GaAs SPDT switch IC's. Isolation of the switch IC's is improved by 5 dB at 3 GHz by the subsidiary inner ground leads. The isolation characteristics are discussed based on the equivalent circuit extracted from the simulation results.