Due to the capability of solving the band crowd and good performance in electromagnetic countermeasure, passive radar using illuminators of opportunity is an active research area in recent years. Taking into account the fact that most illuminators of opportunity transmit narrowband or ultra-narrowband Continuous Wave (CW) waveforms, a novel image formation method is presented for passive synthetic aperture radar (SAR) using narrowband CW waveforms. First, the data model is built up for imaging by correlating the signals received from different airborne receivers. Then an image formation method is presented using the Filtered-BackProjection (FBP) technique. Various potential factors that influence the quality of the imaging are analyzed. Finally, numerical simulations demonstrate the performance of the echo-correlation-and-FBP based passive SAR imaging method. This method does not require a priori knowledge about the transmitter locations and is applicable to the case where the radars are travelling along arbitrary flight trajectories.