A power factor improvement circuit includes a low frequency filter unit installed between two electrodes of an output terminal of a rectifier unit for adjusting voltage and current inputted to a PWM control IC in-phase, and first and second compensation circuits installed at a current compensation terminal and a voltage compensation terminal of the PWM control IC respectively, and the first and second compensation circuits are provided for reducing the current gain of the phase adjustment unit to avoid any unnecessary action of the PWM control IC, so as to achieve the effect of controlling a power factor to a level over 0.90 when a full voltage of 90-264V is inputted.