Onion production and productivity is constrained by biotic and abiotic factors. Soil salinity is one of the factors mentioned by producers in Ethiopia. Hence, the study was conducted to determine the relative tolerance of onion cultivars to salt stress levels under field condition on soil media filled in box at Melkasa Agricultural Research Center. Factorial combined treatments (4×5) were arranged in Randomized Complete Block Design with three replications. Data collected were subjected to analysis of variance using SAS version 9.3. The analysis revealed variation in growth, yield and yield related parameters among cultivars (p≤ 0.05) and salt stress levels (p≤ 0.001). Bombey Red, Nafis and Nasic Red cultivars showed the highest performance in number of leaves per plant, pseudo stem diameter, plant height, leaf length and width, fresh and dry above ground weight. The cultivars also showed the highest bulb length, width, fresh and dry weight. Chlorophyll and stomatal conductance was highly significant for main factors and interactions (p≤ 0.001). Photosystem-II also showed significant difference at 40th and 68th (DAT) days after transplanting among cultivars (p≤ 0.05) and at 40 and 54 DAT stages within salt levels (p≤ 0.001). With 1.2 dSm-1 salt level, the highest growth and yield performances were observed. The highest photosystem-II was recorded for Bombey Red, Nafis and Nasic Red, whereas Adama Red showed the least. Generally, during early growth stages, growth variables were not affected up to 4 dSm-1. Thus, Bombey Red, Nafis, and Nasic Red can be used for salt levels less than 4 dSm-1. Indeed, the experiment should be repeated under controlled environment with multi-cultivars and salt levels not more than 4 dSm-1.