We propose a novel physical mechanism for breaking the diffraction barrier in the far field. Termed fluorescence emission difference microscopy (FED), our approach is based on the intensity difference between two differently acquired images. When fluorescence saturation is applied, the resolving ability of FED can be further enhanced. A detailed theoretical analysis and a series of simulation tests are performed. The validity of FED in practical use is demonstrated by experiments on fluorescent nanoparticles and biological cells in which a spatial resolution of <λ/4 is achieved. Featuring the potential to realize a high imaging speed, this approach may be widely applied in nanoscale investigations.