Primary-side sensing (PSS) technique can be used for output voltage or current regulation by employing an auxiliary winding for the sensing of the output voltage without using an additional opto-coupler for output feedback isolation. However, the sensed voltage may be corrupted by switching noises due to leakage inductance, winding resistance, uncertainty of winding parameters, and nonlinearity of the magnetic core, therefore, the sensed voltage can only achieve limited accuracy. This paper makes an error analysis of the PSS technique due to the leakage inductance and other key parameters when applying to flyback converters. Computer simulation has been carried out to verify the proposed error analysis method. The developed error analysis method can be used to improve the design of the flyback transformer and PSS algorithm for the development of more advanced digital primary-side controller in applications to flyback converters.