The effect of transistor capacitance on the frequency, gain and amplifier efficiency has been studied. It is known that the maximum frequency of an amplifier on HEMT decreases with a decrease in the gate-source and gate-drain capacitance. The effect of the transistor inter-electrode capacitances on frequency, the power gain and the efficiency has been estimated. In the work the amplifiers circuits simulation based on HEMT, manufactured at MIET, has been performed. The transistor has been made on the basis of GaN with 0.25 µm gate, the EEHEMT model has been used. The amplifier maximum frequency was 59 GHz. A 20% decrease in the gate - source and gate - drain capacitances increase the maximum frequency up to 90 GHz. The transistor capacitance affects the power consumed from the input source. This is due to the direct power passage from the input source to the amplifier output. The simulations, carried out at the 4 GHz frequency, have shown a decrease in the input power, supplied to the transistor, while reducing the input and passage capacitances. With a 20% decrease in capacitance the input power has decreased by ~ 2dB, which leads to an increase amplifier Pgain and PAE. A method for reducing the capacitances of a HEMT with a T-gate has been proposed. After manufacturing the transistor the SiN dielectric layer between the gate metal and the AlGaN layer should be deleted. Removing the dielectric layer leads to a decrease in the relative permittivity of the space between the gate and the transistor channel with a corresponding decrease in capacitances. Since the capacitances of the transistor structure are an essential part of the total capacitances (up to 40%), this method leads to the decrease in the inter-electrode capacitances, which makes it possible to implement the simulation conclusions. The result of the paper can be useful for optimizing the HEMT design with T -gate.