The effects of high-temperature treatment for enhancing budbreak in ‘Delaware’ grapes grown under forced conditions in Shimane Prefecture were investigated. Cuttings (10 cm) with one bud each were treated with high temperature at 35°C and 40°C in November, December, and January. Treatments for 48 h at 35°C and 24–48 h at 40°C led to a significantly higher budbreak rate than in controls (23°C 12 h-L/18°C 12 hD). Additionally, the number of days required for budbreak (NDRB) increased when treated at 25°C in November and December compared to treatments at 30°C in November and 35°C in December. On the other hand, NDRB for 24 and 48-h treatments in December and January were lower than for 8-h treatments. Treatment at 40°C also resulted in a lower NDRB than that at 35°C, where the NDRB with intermittent treatment (heating for 4 h/d) was higher than that with continuous treatment. The NDRB on treatment at 25°C in November and December was higher than on treatment at 30°C in November or 35°C in December. NDRB with 24-h treatment was also higher than that with 48 and 64-h treatments in November. Lastly, the NDRB was lower if the high-temperature treatment was performed for longer. These results suggest that high-temperature treatments may promote budbreak and may also be dependent on the exposure period and temperature.