In this study a convective dryer was used to dry two types of onion (red and white) under three different thickness (0.5,1 and 2 cm) and two different levels of air temperatures (65 and 80 oC) to esteem drying characteristics, quality evaluation and the effective moisture diffusivity. At the end of the experiments, the samples were reached 8±1%, w.b. moisture content. The effect of air drying temperature was respectable on the drying time. The Page and Modified Page (I) models were the best compatible models for all treatments. For dried samples quality assessment, the total color change (δE) was better in the dried slices at lower temperature in both types for all slices thickness. The rehydration ratio extended from 2.03 to 2.68 at different parameters. Generally, the rehydration ratio was preferable at low temperature with regard to the two types of dried onions. From the results of quality assessment, convective drying has no defect to the dried samples, so that it is one of the best ways to dry onions. Effective moisture diffusivity of white onion extended from 1.50×10-7 to 4.90×10-7m2.s-1, however red onion, extended from 6.25×10-7 to 6.50×10-6m2.s-1. The moisture diffusivity increased as drying air temperature increased for both types of onion.