The combination of outstanding properties, such as high thermal conductivity, high refractive index, extreme hardness, radiation resisting, and high insulation, makes diamond an ideal material for optical application under extreme requirements. The relationship between the optical absorbance of diamond doped with nitrogen and the nitrogen concentration were studied. The calculation method of nitrogen concentration used in this study was a modification of the typical calculation method. The high nitrogen concentration diamond was synthesized in Fe80Ni20-carbon and sodium azide system. The absorbant intensity of diamond increases in 800—1400 cm-1 range. The nitrogen concentration in diamond increases with the increasing contents of NaN3. The color of diamond changes with the increase of NaN3 content, in the order of green, dark green and black. The diamond synthesized with addition of NaN3 contains nitrogen exceeding 1450 ppm which is much higher than the normal diamond. The optical transmission of diamond decreases with the increase of nitrogen concentration. The diamond with nitrogen doping can be used as optical material with better absorbance and many physical characters at some wavenumber.