Abstract: Co-PAN complex was formed by the reaction of 1-(2-Pyridylazo)-2-naphthol and cobalt(Ⅱ) ion. The mixture was filtered with membrane,then visible diffuse reflectance spectra of the membrane filters were acquired. Based on the fact,a method for the determination of trace amount of cobalt in pond water and vitamin B12 tablets by visible diffuse reflectance spectrometry with membrane filtration enrichment was proposed. Linear relationship between values of absorbance and mass concentration of cobalt was obtained in the ranges of 0.2-3.0 μg·L-1 and 3.0-10.0 μg·L-1,with detection limit (3s/k) of 0.093 μg·L-1. The proposed method was applied to the analysis of samples of pond water and vitamin B12 tablets,giving values of recovery and RSD′s in the ranges of 96.7%-105% and 1.4%-2.9%,respectively.