Abstract: Wavelength calibration needs to be done before the spectral measurement of a monochromator. The relationship between the output wavelength and the number of stepping motor pulses of Czerny-Turner double grating monochromator was derived. Experiments of wavelength calibration were carried out. A low pressure mercury lamp was used to provide characteristic calibration lines at ultraviolet and visible wavelength range, spectrum scanning was done by the monochromator to get the data of signal intensity versus indicated wavelength. Finite wavelength correction was computed from the data. Then correction at arbitrary output wavelength ranging from 250nm to 450 nm could be obtained using curve fitting. The wavelength calibration insured the accuracy of the monochromator wavelength.