Abstract: Since 1889, the international prototype of the metre has served as the definition of the unit of length in the International System of Units. In 1960 the definition of the metre was changed, using a spectral line of 86Kr; in 1983, this was again redefined using the speed of light in vacuum and optical frequency. This will influence the definition of several other base units based on other fundamental physical constants. In this paper, the principle and experimental results of precise measurements of the speed of light in vacuum is presented. These results are of important significance for physics and metrology.