Abstract: Global warming could affect phenology of trees by modifying the bud break dates and consequently the growing season, which would become much less adapted to their local environment. Six plots of Quercus suber (from 35 to 50 trees per plot) were monitored from the beginning of March to the beginning of June 2018 at the level of two bud break dates (start and full bud break) along an altitudinal gradient ranging from less than 100 m to 800 m in Kroumie-Mogods in the North-West of Tunisia. Our data showed that in the sites located at low altitude, the two types of bud burst were recorded respectively between the end of March and mid-April. On the other hand, at higher altitudes, bud burst was delayed of 2-3 weeks (April-early May). A positive correlation was obtained between the dates of bud break and the altitude. The phenological differences in the response of trees to temperature suggest that global warming will significantly affect the distribution of this species which represents a heritage plant of great importance in Tunisia.