Effects of seed size and osmotic stress on germination and seedling growth of five naked oat genotypes were investigated in this study. Oat genotypes were Mozart, CROA 60, Detvan, Eva 1 and AC Belmont. Small, medium and large seeds of five naked oat genotypes were germinated in polyethylene glycol (PEG 6000) solutions with initial osmotic potentials ranging from 0 to -0.75 MPa at 8 degrees C. Final germination percentage, median germination time, root length and shoot length were measured in this study. In all genotypes examined, decreasing seed size and osmotic potential increased median germination time (MGT) and decreased final germination percentage (FGP), root and shoot length. Among genotypes, CROA 60, Eva 1 and AC Belmont had the highest final germination percentage while AC Belmont had the fastest median germination time and the highest root and shoot length. The study results also show that large seed of genotype such as AC Belmont seems better fitted to germinate under the range of osmotic potential in this study.