Rice plants were grown in artificially illuminated growth cabinets under conditions of ambient CO₂ concentration, 28/23°C, 12-hr day/12-hr night temperatures, and 3 or 300μM phosphorus in a standard nutrient solution with soil. At the 10-leaf stage, atmospheric CO₂ (350 or 700μmol mol^-1) and temperature (28/23 or 28/28°C) were controlled and maintained until the harvest at 40 days after heading. At 28/23°C, the high phosphorus accelerated heading by 2-3 days while the high CO₂ accelerated heading only by one day. Culm and panicle lengths were not affected by high CO₂, but they increased under high phosphorus. Panicle numbers were greater in both high CO₂ and high phosphorus. Panicle weight increased 20 and 40% at low and high phosphorus, respectively, in the high CO₂ treatments without amelioration of the harvest index (ca. 0.5) . In 28/28°C-high CO₂ plot, the heading was accelerated 3-4 days and the plant weight was similar to 28/23°C-high CO₂ plot, but the harvest index was lower (<0.4) than for the 28/23°C-low CO₂ plot, because the dry matter was preferentially partitioned to vegetative parts, especially to roots. At elevated atmospheric CO₂, the phosphorus nutrition is important to increase rice yield. Thus, under the expected climate change due to increased green-house gases, appropriate plant nutrition to control photoassimilate partitioning will be an important practice.