Abstract: Mushroom sample was digested with HNO3,fumed with HClO4 to decompose organic materials and then treated with HCl to liberate selenium as Se(Ⅳ) ions.Selenium(Ⅳ) in the slightly acidic sample solution was reacted with 1,2-diaminonaphthalene to form 2,3-naphtho-1,3-selenodiazole which was extracted into cyclohexane.The fluorescence intensity of the fluorescent product in cyclohexane was measured at λex of 375-380 nm and λem of 520 nm in 1 cm quartz absorption cell.Linear relationship was kept between fluorescence intensity and concentration of selenium(Ⅳ) in the range of 0.001-0.10 mg·L-1.