The photodegradation of 3, 3-diindolylmethane(DIM), which has the antifungal activity against Xanthomonas campestris pv. oryzae, Cochliobolus miyabeanus and Pyricularia oryzae, was investigated under xenon lamp irradiation. The maximal degradation of DIM in 12.5 ppm methanol solution was observed in the UV range of 280 nm to 290 nm. The range of wavelengths for the maximal degradation shifted to longer wavelengths as DIM concentration in methanol solution increased. For example, the range shifted from 280-290nm in 12.5ppm solution to 320-330nm in 200ppm solution. Nine compounds were detected as the photodegradation products of DIM and their structures were tentatively determined by gas chromatography-mass spectrometry as 3-formylindole, indole, 2-aminophenol, indole-2, 3-dione, methyl 2-aminobenzoate, indole-2-one, 3-methoxymethylindole, methyl 2-ethylaminobenzoate, and 1-(2-dimethylaminophenyl)ethanone, respectively.