Highlights The impact of kernel thickness grading on seed germination and vigor was explored in medium and low moisture content seeds using two hybrid rice cultivars (RTv7303 & RT7301). Rough rice samples were size fractionated into two thickness fractions (thick >2.03 mm and thin 1.91>>2.03 mm) and dried to 12.5% moisture content (MC) at temperatures of 40°C and 60°C and 25% relative humidity (RH). It was discovered that harvested rice seeds at a medium MC (19% to 21%) and a temperature of 60°C increased seed germination and vigor. Thick kernels outperformed thin kernels in terms of seed germination but were not statistically different when compared to unsorted kernels. ABSTRACT. Rough rice lots harvested on the field contain varying kernel thicknesses within the panicles. These variations influence the drying characteristics of seeds and, thus, the seed germination and vigor. In this study, two hybrid long-grain rice cultivars (RTv 7303 & RT 7301) with harvest moisture contents (HMC) ranging from 19% to 21% and 15% to 17% wet basis (w.b.) for medium and low MC seeds were used, respectively. These rough rice samples were size fractionated into two thickness fractions (thick >2.03 mm and thin 1.91>>2.03 mm), with non-fractionated samples labeled as unsorted or control samples. Kernels from each sub-lot were dried to 12.5% MC at 40°C and 60°C air temperatures and 25% constant relative humidity (RH). Immediately after drying, samples were selected for seed germination and vigor assessments. RTv 7303 had higher seed germination compared to RT 7301, indicating that the two cultivars were genetically different from each other. Thick kernels had significantly higher seed germination (SG) than thin kernels, but there was no significant difference compared to unsorted kernels. Additionally, the percentage points difference between the thick versus unsorted and thin kernels varies from ˜1-pp to 8-pp for low MC seeds and from 2.5-pp to 6-pp for medium MC seeds. The seed vigor index (SVI) difference between the two cultivars is around 3.0 m.% and 1.5 m.% for medium and low MC, respectively. Harvested rice seeds with a medium MC (19% to 21%) and dried at a temperature of 60°C resulted in greater seed germination and vigor. Ultimately, these findings highlight the role of thickness grading in seed assessments. Thus, it was concluded that if the thin kernels could be screened out from the harvested rice lots, seed germination and vigor would increase significantly. Keywords: Rice drying, Rice seed, Seed germination, Seed vigor, Thickness grading.