In this paper, a new transition between GaAs chip (Microstrip) and Aluminum motherboard (Microstrip) with a single interconnect and one common ground has been analyzed and investigated using Finite-Difference Time-Domain method. The objective is to optimize the package performance over a wide frequency band up to 50 GHz. This is carried out by performing a parametric analysis to study the effects of the transition interconnect (discontinuity) on the overall package performance. The scattering parameters of interconnect are used as a performance measure of package  under  investigation. Good results have been obtained up 50 GHz; S12/ S21 and S11/S22 are about -0.4 and -20 dB respectively.