The photoisomerization of piperine solution were investigated by different irradiation conditions such as different time,wavelength and light source.Piperine and its isomers were determined by HPLC.The results showed that(1) piperine and its isomers inter-converted and achieved dynamic balance after irradiated for a certain period of time.(2)The effect of piperine radiated at 365 nm on photoiaomerization was higher than that of 254 nm.(3) The lost of piperine were almost the same when piperine were irradiated with different light sources.UV light benefited the format of chavicine and isopiperine,while sunlight helped more isochavicine being formatted.These results could provide a reference for studying the stability and the retention rate of piperine.