A simple method was studied for determination of selenium in water. Fifteen ml of nickel ammonium sulfate solution (100mg/l) as coprecipitant was added to sample solution. Then 10ml of potassium hydrogen phthalate buffer solution (5 %) was added to the sample water, and pH of the solution was adjusted to 4 with hydrochloric acid. Five ml of sodium diethyldithiocarbamate solution (2 %) as precipitant was added to the sample water. The sample water was allowed to stand for 30 minutes for aging the precipitation. The precipitate was collected on a membrane filter paper (0.45μm), air-dried and the fluorescent X-ray analysis was carried out. The recovery of selenium with the proposed method was higher than 97%. The lower limit of determination of the method was 1.9μg/l when 1000ml of sample water was used. This method was applied to the analysis of selenium in waste water, and the obtained result agreed with that of JIS method. (Proposed ; 5.1μg/l, JIS ; 5.0μg/l)