Ultraviolet (UV) light induced photo-degradation of malachite green (MG) was investigated in the absence of photocatalysts. Photo-degraded intermediates were identified using LC with visible detection and LC-electrospray ionization-MS. Using this analysis, mechanisms for the MG decomposition in aqueous solution and in mudfish tissue were suggested. In aqueous condition, N-de-methylation and the decomposition of the conjugated structures were mainly observed. On the other hand, MG levels in mudfish continuously decreased until they reached a plateau owing to the decomposition of the conjugated structures. These results show that some chemicals in fish tissues could be photo-degraded by UV light without photocatalysts.