Onion seeds, cv. Aurora, were primed using aqueous solutions of PEG 6000 in the osmotic potentials of -0.8, -1.0, -1.2 and -1,4MPa and soaking periods of zero, 24, 48, 72 and 96 hours, at 20°C of temperature and constant aeration. After each treatment, seeds were washed, dried superficially, weighted and dried in stove with circulation of air for 30°C until reaching initial moisture content. The tests used for evaluating the seeds were: germination test, cold test, emergence at green house, shoot plant length, dry matter weight of the shoot plant, emergence speed, radicle length and seedling dry matter weight. Osmoconditioning with PEG 6000, propitiated increments on plant growth, on shoot and radicle Iength and biomass accumulation. Germination and vigor of onion seeds decrease as the osmotic potential increase from -1.0MPa until -1.4MPa and the imbibition period form 24 until 96 hours. Onion seeds primed with PEG 6000 aqueous solution, at -1.0MPa for 24 hours of soaking, show higher germination and vigor, uniform and faster emergence, larger radicle and plant growth, and larger biomass accumulation.