The use of feedforward correction to reduce nonlinear distortion in a microwave amplifier is investigated, using the Volterra series approach. Expressions are derived for the suppression of main amplifier distortion, in terms of the circuit parameters of the system. It is shown that the gain and phase imbalance in the second interference loop mainly determine the distortion suppression in the feedforward amplifier. An expression is derived for the relationship between amplifier distortion suppression and the degree of gain and phase imbalance in the second interference loop. The effect of the phase imbalance in the second interference loop on the AM-to-AM and AM-to-PM conversion in a feedforward amplifier is discussed; it is shown that the presence of such phase imbalance may cause a substantial difference to exist between the improvement in cross-modulation and that in intermodulation. Further, the gain sensitivity of the feedforward amplifier as a function of gain and phase imbalance is obtained. As an illustrative example, the distortion suppression of a specific IMPATT amplifier with feedforward correction is considered.