Abstract: In a borate buffer medium of pH 8.05 and in the presence of Tween-60,mercury(Ⅱ) was reacted with p-nitrodiazoaminoazobenzene (p-NDAAB) forming a stable red complex,having its absorption maximum at the wavelength of 480 nm and apparent molar absorptivity of 1.54×105L·mol-1·cm-1.Beer′s law was kept in the range within 0.7 mg·L-1.The proposed method was applied to the analysis of waste water samples,values of mercury contents found were checked quite well with the results found by the dithizon-photometric method.