This study was conducted during 2011 and 2012 at the experimental farm of ICAR Research Complex for Eastern Region, Patna, India in Open top chamber (OTC) as well as in open field condition. Four rice genotypes (Local/Indigenous:Rajshree, promising HYV of state: Rajendra Bhagwati, promising HYV of eastern region: Swarna sub 1 and ruling variety in state: MTU 7029) were used for the study. The 1 hr averages of CO2 in the elevated CO2 OTC ranged between 455 and 510 ppm with a mean of 482 ppm and a standard deviation (S.D.) of 12.7ppm. LAI of the elevated CO2 treatment was significantly ( p<0.05) greater than the control OTC and open field at all times in both kharif seasons. The higher grain yields in rice crops under elevated CO2 were primarily due to their greater biomass production capacity and not due to a greater fraction of total biomass being partitioned to grains. The greater biomass under elevated CO2 was brought about by an increase in RUE, during both pre- and post-anthesis periods. In both the years, rice growing under elevated CO2 had significantly greater grain numbers and grain yields than under ambient CO2 and open field conditions, which did not differ significantly. The percentage of filled grains also was significantly greater in the elevated treatment.