A discrepancy of monolithic microwave-integrated circuit (MMIC) amplifier simulations is discussed for conventional electromagnetic (EM) circuit co-simulation in advanced design system (ADS). An obvious discrepancy occurs when microstrip (MS) type transistor schematic models are used in a high gain MMIC amplifier. The coupling effect of a backside via hole in the MS-type model, which is excluded from EM layout simulation, is demonstrated to be the root of this error that will become significant in high-gain MMIC amplifier design. A new method is proposed to include a source via hole of the MS-type transistor in MMIC layout EM simulations. Simulation experiments confirm that this new method can significantly improve the EM simulation accuracy when including a transistor source via hole in the circuit layout EM simulation. The intercoupling effect of the transistor source via is investigated on the correlation with frequency, current gain, via separation distance, and substrate thickness.