An ion chromatography method with non suppressed conductivity detection was developed for the simultaneous determination of trimethylamine N oxide (TMAO) and its decomposition products (trimethylamine, TMA, and dimethylamine, DMA) in fishery products. Fishery samples were treated with 10% trichloroacetic acid, afterwards the extracted solution was centrifuged and diluted before injection to the ion chromatography device. The targeted compounds were eluted from a Metrosep C4 150 (150mm×4mm) column by 4mmol/L nitric acid with 3% acetonitrile (v/v). The retention time of DMA, TMAO and TMA were 6.22min, 8.21min and 9.09min, and the detection limits were 0.025mg/kg, 0.050mg/kg and 0.050mg/kg, respectively. The coefficients of linear equations were all above 0.999. The RSD were between 1.41% and 3.83% (n=5), and the recoveries were between 91.5%~100.3%. DMA was found in the samples which contain endogenous formaldehyde, while sample without endogenous formaldehyde was DMA free.