This research aimed to examine the effects of combining increased temperature and carbon dioxide (CO 2 ) under RCP 4.5 scenarios on yield and seed chemical composition of Thai rice cultivars; Chinat1, Phathumthani1, and Phitsanulok2. The field warming experiment was conducted at Phitsanulok from 2018 to 2020. Sixteen field-open top chambers (OTCs) were applied to simulate the expected future climate situation under RCP 4.5 (the elevated temperature at 2°C and CO 2 up to 800 ppm, respectively). The results revealed that the single factor of elevated temperature at 2°C led to statistically significant grain size reductions of 2 rice cultivars; Chinat1 and Phitsanulok2. We also found that the combination of elevated temperature and CO 2 induced a significant grain yield loss (ton.ha -1 ) by 68% in Phitsanulok2 but an increase of 38% appeared in Chainat1. Considering results in seed chemicals, the statistically significant decrease trends in Amylose and Brix values were shown in 3 cultivars. These results suggest that elevated temperature and CO 2 caused the imbalance in seed nutrient contents and an increase in yield loss in some Thai rice cultivars.