Onion is an essential and familiar spice as well as vegetable crop throughout the world. It is a semi-perishable vegetable and inappropriate agricultural and postharvest practices usually result in a higher loss in many developing countries including Ethiopia. This study was carried out in Northwest Ethiopia in the year 2019 aiming to assess postharvest loss of onion along the supply chain (from farmer to retailer) using FAO‟s load tracking approach. Three growing locations (Gumara, Andasa and Merawi) were purposively selected and onion samples from these locations were followed at wholesaler, and retailer level at Bahir Dar market. The postharvest losses of onion were investigated by assessing the percentage of quantitative loss (rotting, mechanical damage and sprout bulb) and changes in physicochemical properties (total soluble solids, dry matter content, color, physiological weight loss, firmness and pyruvic acid) along the supply chain. Statistical analysis was performed using two- way ANOVA by SAS version 9.2. The result shows that most of postharvest loss of onion was due to rotting and mechanical damage. This was in turn caused by lack of access to appropriate tools and skills during harvesting and inappropriate postharvest handling (sorting, curing, packaging and storage) practices. The highest (30.9%) postharvest loss was observed in Andasa with the highest (21.2%) loss was due to rotting at the wholesale level. The average loss of 20.7 and 22.8% were obtained for Gumara and Merawi, respectively. On the other hand, the chemical composition of onions from the three areas (total soluble solid, dry matter, color, firmness and pyruvic acid content) had no statistically significant changes along the supply chain. The use of appropriate tools and skills during pre and postharvest activities is vital to reduce postharvest loss of onions.