In this study, a solar tunnel dryer of 6 x 3x 2.7 m (LxWxH) constructed with galvanized frame structure and covered with 200 µ UV-stabilized LDPE sheet. Two fresh air inlets of 0.6 x 0.3 m size and two exhaust fans each of 9'' diameter and 50 watt capacity were installed at the rear side and front side of the dryer, respectively. The onion slices pre-treated with 0.25 % potassium meta-bi-sulphite and untreated slices were subjected for drying. Results obtained from this experiment showed that in solar tunnel dryer, the drying required 12-13 h for various pre-treated onion samples to reduce the initial moisture content ranging from 797.29 to 572.92 % (dry basis) to final moisture content in the range of 6.24 to 6.08 % (dry basis) and in open yard sun drying, it required 14-16 h for various pre-treated onion samples to reduce from moisture content of 772.29 to 567.15 % (dry basis) to safe storage moisture content of 6.29-6.22 % (dry basis) at different drying air temperatures and relative humidities. Two drying models were fitted to experimental drying data of onion slices namely Page and Midilli-Kucuk model and Midilli-Kucuk model was found to be the best fit to describe the drying behavior of onion slices. The use of solar tunnel dryer leads to considerable reduction of drying time in comparison to sun drying. The onion slices being dried in the solar tunnel dryer were completely protected from rain, insects and dust. The dehydrated products could be well utilized for substituting the fresh product in off season.