An increase in temperature is predicted to have a negative effect on plant growth and development, thereby leading to its loss of yield. Higher air temperature has damaging effects on plant physiology, including its photosynthetic mechanism. Therefore, this study aims to investigate the effects of short-term high temperatures on photosynthesis at PSII, and the growth of rice cultivars at the heading stage. The experiment was carried out with two rice cultivars, namely, PT60 and Dular under 45°C for 30 minutes. The results showed that panicles and leaf tips of rice cv. PT60 had chlorophyll bleaching, and at day 5 after heat treatment (DAH), the symptoms were very severe. On the other hand, all plant parts of cv. Dular remained green at 5th DAH. Fluorescence parameters in cv. Dular remained unchanged after heat treatment. In contrast, cv. PT60 exhibited a significant decrease in all fluorescence parameters. Also, the electron transport rate in cv. PT60 drastically declined after exposure to heat. The growth of cv. PT60 was inhibited by heat stress as indicated by a slight reduction in plant height, whereas cv. Dular continued growing after heat exposure. Therefore, the cultivar PT60 was susceptible to heat, and the cultivar Dular seemed to be tolerant to heat during the heading stage.