Abstract: The concentration of fluorine in rock samples was determined by spectrophotometry in p-sulfophenylazo chromotropic acid (SPADNS) system. The best determination conditions are: measurement wavelength of 570 nm, determination media of 0.84 mol/L HCl, chromogenic reagent (SPADNS) dosage of 5.0 mL and reaction equilibrium time of 2 h. The Beer′s law was obeyed in 0～1.4mg/L of F- with detection limit of 0.06 μg/g. The interference from coexistent elements could be effectively reduced by using standard rock samples for calibration instead of using standard fluorine solutions. The method has been applied to the determination of fluorine in National Standard Reference Materials and Japan Standard Reference Basalt sample. The results are in agreement with certified values with precision of 2.37%～6.32%RSD (n=6).