Primary-side sensing (PSS) technique can be used for the output voltage or current regulation of a flyback converter by employing an auxiliary winding for the indirect detection of output voltage and conduction period of the output rectifier diode. However, errors in the estimation of output voltage or current may be induced by the voltage drop of the secondary diode, secondary leakage inductance, secondary winding resistance, and measurement error due to limited bandwidth of the sensing circuits. In this paper, a sampling instant modulation technique is developed to reduce the sensing error in primary-side sensing of flyback converters. This new PSS sampling technique is then implemented for the power factor correction (PFC) of a flyback converter. A multi-mode control strategy can improve sampling accuracy at the zero-crossing of the rectified input voltage. Experimental results are shown to verify the feasibility of PSS technique realized with digital control technique.