Bad　weather,　such　as　fog　and　haze,　can　significantly　degrade　the　imaging　quality,　which　becomes　a　major　problem　for　many　applications　of　computer　vision.　In　this　paper,　we　propose　a　novel　color-preserving　defog　method　based　on　the　Retinex　theory,　using　a　single　image　as　an　input　without　user　interactions.　In　the　proposed　method,　we　apply　the　Retinex　theory　to　fog/haze　removal　form　foggy/hazy　images,　and　conceive　a　new　strategy　of　fog/haze　estimation.　Experiment　results　demonstrate　that　the　proposed　method　can　not　only　remove　fog　or　haze　present　in　foggy　or　hazy　images,　but　also　restore　real　color　of　clear-day　counterparts,　without　color　distortion.　Besides,　the　proposed　method　has　very　fast　implementation.