Dormex at 0, 2, 3 and 5% was sprayed on 1 or 7 or 14 January during two seasons 2006 and 2007 on 15 years old Thompson Seedless grapevines, planted at 2 X 2 meter in a loamy clay soil on three dates. The results of the two experimental seasons indicated that, the response of Thompson Seedless grapevines to (Dormex) treatments varied according to the concentration and spraying date. Budburst percentage generally increased from the basal to distal of bud location. The percentages of budburst, time budbreak and the vegetative growth were markedly increased respectively compared to control. There beneficial results concerning an advancement in date of bud burst, an increase in the percentage of buds developed to vegetative growth and appearance of flowering. The best results were obtained by 5% hydrogen cyanamide applied on the first of January in both season.