In this paper, the effect of gm-mismatch on the performance of Doherty power amplifier is studied. For this purpose, the mismatch effect is mimicked by driving the main and peaking amplifiers, asymmetrically. Three mismatch scenarios corresponding to the cases where the peaking amplifier is overdriven, normal driven and underdriven are considered. Both simulation and experimental results clearly revealed that the amplifier's efficiency and linearity strongly depend on the drive levels of the main and peaking amplifiers. Increasing the drive level of the peaking amplifier improves the amplifier's linearity, but sacrifices its efficiency. This fact indicates that drive levels of peaking and main amplifiers, hence g(m)-mismatches is a crucial factor that should be taken into consideration carefully in the design of Doherty power amplifiers. Experimental results of a 40 dBm Doherty amplifier operating at 2645 MHz showed that IMD3 of -24.83 dBc measured when peaking amplifier is overdriven is reduced to -21.16 dBc for the underdriven case. Nevertheless, the drain efficiency which is measured at 40.39 % when peaking amplifier is overdriven is improved to 43.8 % in the underdrive operation.