A method for correcting gain imbalance error, phase imbalance error and DC offset errors in a transmitter having an OFDM-based I/Q modulator is disclosed. The method employs a compensator prior to the I/Q-modulator to compensate for the gain and phase imbalance and DC offset. The compensator is efficiently updated with the estimated values of gain and phase imbalance and DC offsets obtained by performing the DFT operation in the digital baseband domain while sending a pair of orthogonal test tones to the modulator's inputs from a digital baseband chip, then down converting the RF modulated signal through a nonlinear device and a bandpass filter to a baseband signal, and finally sampling it using an A/D. The delay mismatch, which is mainly generated by lowpass filters between the I and Q branches, is also minimized in this method.