Onion is one of the most important crops in the world and also in Iran. In spite of its high quantity produced in Iran, it sent to the domestic and foreign markets in inappropriate conditions, and at a low price due to the lack of sorting process and appropriate packaging. The aim of this study was to test and evaluate the onion sorting machine, study the effect of performance characteristics of the machine on the sorting quality indexes and determine the probable mechanical damage to the product during the sorting in order to achieve the optimum performance characteristics of the machine. This machine was designed and constructed with two reciprocating sieves with a length and width of 2*1 m, using a wobble shaft. Onion was sorted at 3 degrees based on their diameter according to the international standards code (D<50 mm, 50mm70mm). The sorting performance indexes consist of the degree of pollution and the degree of purity were measured and calculated during sorting of 200 kg of onion. Also, based on the obtained indexes in all test conditions, the amount of optimum performance characteristics of the machine was determined such as effective capacity, slope and frequency of sieves and their effect on the onion mechanical damage during the sorting process. The highest degree of purity equal to 95.60% was obtained at a frequency of 3.33 s-1 and a slope of 10 degrees for the sieves at a capacity of 1 to 1.2 t/h. Also, the overall sorting efficiency of the machine was 92.34% and no mechanical damage was observed to the onion.