Drought, as an abiotic stress inflicts destructive and harmful effects on plants at their different growth stages. Plants react differently to drought. Among their reactions is the production of such osmolytes as Proline and Glycine Betaine. It seems that, through exogenous application of these materials, plant resistance to drought stress can be increased. Therefore, a factorial experiment, based upon a randomized complete block design of four replications was performed on four grape cultivars (Khushnav, Peykani, Perlette, and Flame Seedless). Grapevines, under drought stress, were irrigated by 70% of the required water. Proline (10 mM) and Glycine Betaine (15mM) were sprayed on the grapevines at four growth stages (before flowering, flowering, sour cluster and veraison). Results indicated that Proline and Glycine Betaine significantly (P<0.01) affected leaf Relative Water Content (RWC), canopy temperature, chlorophyll content, leaf area and soluble carbohydrate in all the treated cultivars. A comparison of means indicated that the highest soluble carbohydrate content plus leaf area recorded for Glycine Betaine treatment while the lowest canopy temperature recorded for control. Correlation analysis of traits indicated a negative correlation between RWC and canopy temperature while a positive one between soluble carbohydrate and chlorophyll content. Finally, it was concluded that Glycine Betaine was more effective than Proline in increasing resistance to drought stress in grapevine.