The invention relates to a method and arrangement for forming a amplitude and phase modulated transmission signal. One idea of the invention is that he modulation signal is formed of separate baseband signals controlling phase and amplitude modulation. A phase control signal (phi) is used for controlling a voltage controlled oscillator (10), from which there is further obtained a transmission frequency signal for the RF amplifier (5). By means of the amplitude control signal (A), there is controlled the amplification of the RF amplifier. From the output of the RF amplifier, there is created a feedback signal by means of a sampling circuit (6, 7), and said feedback signal is used separately for correcting both amplitude control and phase control, so that the transfer function of the arrangement is made linear. Thus the employed amplifier can be a nonlinear amplifier with a high efficiency, and yet a good phase and amplitude linearity is achieved by applying the described arrangement.