The present invention provides a circuit of reducing electro-magnetic interference for a power converter. The circuit includes an oscillator, a switching voltage divider, and a sample-and-hold circuit. The oscillator has a terminal for receiving a modulation voltage. The modulation voltage is correlated with an input voltage obtained from an input of the power converter. The switching voltage divider is enabled and disabled by a switch to attenuate the input voltage into a sampled voltage in response to a sampling signal. The sample-and-hold circuit receives the sampled voltage to generate the modulation voltage. A switch of the sample-and-hold circuit controlled by a holding signal conducts the sampled voltage to a capacitor of the sample-and-hold circuit to generate the modulation voltage across the capacitor.