Field experiment involving five improved onion (Allium cepa L.) varieties and carried out at Kulumsa Agricultural Research Center during the 2018 to 2019 cropping seasons under irrigated condition to identify the best performing variety for seed production to the target areas of Arsi Zone. The onion varieties included in the field experiment were (Nafis, Robaf, Nasic Red, Bombe Red, and Adama Red). The experimental design was a randomized complete block design (RCBD) with three replications. Phenological and growth parameters, seed yield, and yield components were studied. The result showed that Variety had a significant effect on most phonological and growth attributes as well as on yield attributes. The number of days to flowering and vigorously were significantly affected by variety; while plant height, days to boolting and branch number was not significantly influenced by variety. Flower stalks height and diameter, number of umbel per plant, and umbel diameter also significantly affected by variety. Seed yield per umbel and mean seed yield per hectare also showed a significance difference among varieties. The highest seed yield per hectare (1415.89 kg/ha) was recorded from Adama Red and followed by Nafis variety, whereas Nasic rerd, Bombe red and Robaf show no significance difference among each other, Therefore, it can be concluded that use of the improved onion varieties such as Adama red and Nafis is advisable and could be appropriate for onion seed production in the test area even though further testing is required to put the recommendation on a strong basis.