Effects of seed size and osmotic stress on germination and seedling growth of fifty-five oat genotypes obtained from Europe, North and South America, Asia and Oceania were investigated. Small, medium and large seeds of fifty-five 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. In this study, final germination percentage (FGP), median germination time (MGT), root length, and shoot length were measured and calculated. In all genotypes examined, decreasing seed size and osmotic potential increased median germination time and decreased final germination percentage, root and shoot length. Among genotypes, the highest final germination percentage was obtained with 88.6% from the cv. Evita. The cv. Flamingsplus, Pajaz, Iltis, Lvovskii Rannii, Marta, Auteuil and Samsun (local population) followed with final germination rates of 83.5, 83.4, 83.3, 82.7, 82.1, 82.0, and 80.8%, respectively. Among these genotypes, Pajaz and Lvovskii Rannii also had the fastest median germination time, however, the highest root and shoot length was measured the cv. Centennial. In addition, this genotype also had higher final germination percentage and lower median germination time.