Abstract: Analysis of 0—100mol％ D_2O concentration with infrared spectrophoto meter is based on the fact that heavy water of different concentration has different characteristic spectrum of infrared absorption. The deter mination wave numbers are 3400cm~(-1) for high concentrations of heavy water, 3040cm~(-1), 1900cm~(-1), 1550cm~(-1) and 1450cm~(-1) for medium concentrations of heavy water; 2500cm~(-1) for low concentrations of heavy water. Utilizing heavy water sample of known concentration to draw a standard curve, unknown sample was deter mined by external reference method. Precisions of the method are ±0.02 mol％ D_2O for high concentrations, ±1mol％ D_2O for medium concentrations, ±0.05mol％ D_2O for low concentrations.