The pot experiment was conducted to evaluate heterosis for photosynthesis in flag leaves during ripening stage of hybrid rice variety- Viet Lai 50 (VL50), in compared to its parents (male line-R50, female line-135'). It was observed that VL50 showed significant and positive heterosis over the best parent in terms of CO2 exchange rate (CER) and SPAD value (an indicator correlated with leaf chlorophyll content) as well as specific leaf area (SU). The CER was positively correlated with both SPAD value and SLA, but not with stomatal conductance or transpiration rate. The area of flag leaves of VL50 was higher than that of its parents, however, the flag leaf angle of VL50 was smaller than that of its parents. Cutting flag leaves at the heading time presented a significant decrease of the dry weight of panicles (DWP) in all cultivars during the ripening stage compared to the control treatment (no cutting). However, the reduction of DWP was stronger in VL50 than that in its parents (DWP of VL50 at three weeks after heading reduced to 71 percent, while that of R50 and 135' depressed to 82 percent and 84 percent of the control treatment, respectively). Thus, the larger area of flag leaves and maintenance of high photosynthesis at the ripening stage might be contributed to positive heterosis for grain yield in the hybrid rice VL50.