The effects of the period and temperature of priming on germination and vigor of onion seeds cv. ?Bola Precoce EMPASC 352? were evaluate. The seed samples were imbibed in media containing polyethylene glycol 6000 (PEG 6000) at concentration equivalent to -0,75MPa at 0, 2, 4 and eight days at 10, 15 and 20°C. After this, the seeds were washed and dried superficially and it were evaluated the germination rate and the total germination, radicle lenght, total seedlings emergence, seedlings emergence rate and total germination under stress hidric. The total germination decreased until the period of eight days priming at 10°C. The priming seeds betweeen four and five days of in all temperatures increased the germination rate. The radicle lenght increased in primed seeds at 15 and 20°C. There was no effect of the priming on seedlings emergence and dry matter weight, but the seedlings emergence rate was greater at 10 and 15°C. Under stress hidric condition, the germination rate and total germination of primed seeds were better than unprimed seeds. The priming of onion seeds in osmotic solution PEG 6000 at -0,75MPa and 15°C between four and six days increased the performance of germination and vigor in the laboratory tests.