This study aimed to determine the rice (Oryza sativa L.) yield under double cropping in the Mekong Delta, Vietnam, and identify which climatic factors cause a reduced yield in the wet season (WS) compared with the dry season (DS). A series of field experiments was conducted in the field from 2014 to 2017, along with a supplementary pot experiment, by using the rice variety OM 6976. The yield was 38% lower in WS than in DS, mainly due to a decrease in the spikelet number per panicle and percentage of ripening. This yield reduction observed in WS was most closely related to lower radiation at 51-80 days after transplanting, and was also related to higher temperature. These results suggest that low radiation and high temperature during the reproductive to grain-filling stages were mainly responsible for causing a low yield in WS. In addition, a need to further consider soil reduction was suggested based on more extensive root damage and sulfide levels on roots observed in WS as compared with DS.