Onion is the most important and widely cultivated bulb vegetable in Ethiopia. High postharvest loss during storage is one of the main challenges of onion production and productivity. The present study was therefore conducted with the aim of evaluating the effects of selected pre-harvest treatments on shelf life and postharvest quality of onion varieties in diffused light storage structure. The bulbs of three hybrid (Red coach, Russet, Jambar) and one open-pollinated (Bombay red) varieties produced in Woramit Research Site of ARARI during the 2023 irrigation season using uniform and standard agronomic practices. The pre-harvest treatments were toppling at 70% neck fall with (easy of pulling up) and without irrigation, toppling at 90% neck fall, and harvesting at 70% neck fall without toppling. Moreover, the postharvest parameters of bulbs were collected at 0th 4 th ,8th and 12th week of storage, Accordingly, the treatments were arranged with 4*4*4 factorial combination in RCBD with three replications in the storage experiment. Based on the treatment setup, bulbs were stored in DLS in Zenzelema campus of Bahir Dar University for three months. Data on postharvest quality parameters were collected and analyzed using mixed model combined analysis SAS (version 9.4) software where pre-harvest treatment and variety were considered as fixed variables while storage duration was considered as random variables. The results of combined analysis of variance revealed that variety, pre harvest treatment and storage duration influenced most of the tested parameters of onion in the storage. Similarly, the interaction of variety and pre harvest treatments influenced decay percentage (P<0.001), bulb firmness (P<0.001), total soluble solid of onion bulbs (P<0.0001) and marketable bulbs yield (P<0.001) throughout the storage duration. The varieties Jambar and Russet and the practice of toppling at 90% maturity recorded the lowest sprouting and weight loss percentage of onion bulbs in the storage and highest proportion of marketable bulbs highest bulb firmness. On the other hand Bombay red variety, toppled at 70% maturity followed by irrigation recorded the highest sprouting, decay percentage, bulb weight loss and the lowest bulb firmness. It is conclude that the use of Jambar and Russet varieties and the practice of toppling at 90% maturity are therefore suggested to maintain the quality and reduce postharvest losses of onion bulbs in the storage . As the study is limited to single location and season, it is recommended to repeat the study in multiplications and seasons.