A field experiment was conducted at Horticultural Research Station, Seobag (Kullu) from 2004 to 2010. The phenology of ten apple cultivars was observed on every second day on three plants of each cultivar with three replications from bud-bust, green-tip, pink-bud, full-bloom to physiological maturity. GDD, HTU, PTU, HYTU and heat use efficiency was computed from the meteorological data recorded from observatory and averaged of for ten years. Tydeman, commercial and Mollice has taken lower GDD for the physiological maturity but higher for attaining petal-fall. On an average apple required 330 GDD from bud bust to petal fall with 4o C as base temperature. Cultivars completing the rest period earlier used more thermal units and giving good fruit yields as compare to those breaking their dormancy later. Thermal units explain more than 75-99 % variation in fruit yield. Higher thermal use efficiency was observed for Starking Delicious and Vance Delicious followed by Top Red and Tydeman which may be encouraged among the growers.