Primary-side sensing is an observer-based approach to estimate the output voltage of flyback converters from a primary winding (or an auxiliary winding). Various observer-laws have been recently developed for flyback converters operating in discontinuous conduction mode (DCM). The extension to continuous conduction mode (CCM), however, has not been considered due to the difficulties in compensating for the voltage drop in the secondary winding. From the possibility to predict the winding voltage drop using the magnetizing current and the transformer model, this paper presents a new observer method that can work accurately and smoothly in both CCM and DCM. The methodology can be combined with any controller to provide either output voltage regulation or output current regulation. The proposed sensing technique is verified by simulation.