Seed filling is crucial for seed vigor and starch accumulation and structure. Differences in hybrid rice seed vigor were evaluated in field experiments, conducted across two sites in 2017 and 2018, under different seed filling temperatures along with the underlying mechanisms related to the seed filling characteristics and starch accumulation and structure. Significant differences in the seed vigor parameters were revealed, with different seed filling characteristics observed under different temperatures. When averaged across cultivars, the seeds with a low seed filling rate and long seed filling duration obsessed 11.9% higher germination percentage (GP) and 22.7% higher vigor index (VI) than those with a high seed filling rate and short seed filling duration. Moreover, a high seed filling rate and short seed filling duration significantly decreased the total starch and amylose contents and increased the amylopectin content. Additionally, when averaged across cultivars, the relative crystallinity and starch granule diameter obtained with a high seed filling rate and short seed filling duration were 3.8% and 15.1% higher, respectively, than those with a low seed filling rate and long seed filling duration. In summary, it can be speculated that seed filling characteristics determine hybrid rice seed vigor by affecting starch accumulation and structure. Rice (Oryza sativa L.) is one of the most important crops in China. With increasing human populations and decreasing areas of agricultural land, the promotion of rice production, especially hybrid rice production, is essential in China 1. Seed vigor, which refers to the potential of seed to germinate rapidly and uniformly under a wide range of field conditions, is an essential requirement for agricultural production 2,3. Vigorous seeds of hybrid rice, which are characterized by a fast germination, high gemination percentage and uniform seedling growth, are beneficial for increasing grain yields and profits for farmers 2. In contrast, the hybrid rice seeds with low seed vigor would result in poor crop establishment and lead to great yield losses, suggesting that the seed vigor plays a vital role in the realization of hybrid rice productivity. The seed vigor of hybrid rice can be determined by several factors including seed filling, which is characterized by the duration and rate of seed filling 4. The filling speed of hybrid rice seeds is initially slow, then a rapid filling phase occurs, and then growth slows again with maturity 5. Previous studies have reported that the grain growth of hybrid rice is associated with certain grain filling characteristics 6,7. A high seed filling rate and short seed filling duration limit grain growth and development and contribute to the losses of grain weight and seed yield 7-10. Moreover, Abayawickrama et al. 11 and Liu et al. 12 found that grain quality was affected by grain filling characteristics formed under different temperatures. Therefore, seed filling influences seed growth and development and ultimately affects the seed qu...