Recent studies have shown that rice consumption can be an important pathway of methylmercury (MeHg) exposure to humans in Hg mining areas and also in certain inland areas of Southwestern China. The seed of rice has the highest ability to accumulate MeHg compared to other tissues. The main objective of this study was to investigate the process of (MeHg) accumulation in rice seed (Oryza sativa L.) by monitoring MeHg levels in specific tissues of rice plants experiencing various levels of Hg multisource pollution during a full rice growing season. Four groups of experimental plantations were utilized, distributed among a rural artisanal Hg production site and a regional background control site. Our results suggest that the newly deposited Hg is more readily transformed to MeHg and accumulated in rice plants than Hg forms with an extended residence time in soil, and soil is the potential source of MeHg in the tissues of rice plants. MeHg in soil was first absorbed by roots and then translocated to the above-ground parts (leaf and stalk). During the full rice growing season only a very small amount of MeHg was retained in the root section. In the premature plant, the majority of MeHg is located in the leaf and stalk; however, most of this MeHg was transferred to seed during the ripening period.