Monascus pigment fades in sun light, and ultraviolet radiation(UV) is main reason causing photobleaching of organic compound. To study the mechanism of photobleaching for monascus pigments, a monascus red pigment (resolved in water, molecular formula C19H28N2O5) is radiated with UV-light until colorless, the molecular formula of the colorlessness is determined as C14H9NO6 by infrared spectrometry, mass spectrometry and hydrogen nuclear magnetic resonance. The photobleaching mechanism of the red pigment is deduced according to photochemical theory: firstly, the acyclic side-chain in monascus pigment is cut at α-carbon according to Norrish Ⅰ mode, radicals are formed, and then same reaction happens at 7-carbon methyl; electron-transferring, double-bond conversion and addition reaction between double-bond and hydroxyl radical, superoxide radical from water occurrs, and conjugated double-bond disappeares, which causes bleaching of monascus pigments. In order to avoid bleaching of monascus pigments, stabilization of acyclic side-chain should be considered primarily, and addition of radical clearing reagents in pigments is also prefered.