With the NCEP/NCAR monthly mean reanalysis data from 1948 to 1999, an area index is defined to describe Mascarene high (MH), based on the evolution characteristics of climatological geopotential height at 850 hPa. By calculating the index the features of the MH's variations on different time scales are examined. MH exists all the year round, but it has obvious seasonal variations, the strongest in boreal summer whereas the weakest in boreal winter. An abrupt intensification of MH from winter to summer occurs during May~July, while an opposite weakening from summer to winter occurs during August~November. The variations of the MH's area index have distinct oscillatory components: a 16-year period for the whole period of time, an 8 year period before 1965, and a 3~5 year period after 1967 which is especially significant since 1990s. On the interdecadal time scale,the MH's area index has a remarkable shift in around 1976, indicating that MH is much weaker before 1976 and stronger after it. The relationship between the MH's variation and ENSO is also investigated. It is shown that MH is intensified during an El Nino event, and vice versa. Such a relationship is the best in autumn. Therefore, ENSO is an important factor that can influence the interannual variation of MH.