Application of mixed amine solution in gas sweetening unit decreases the operation cost and corrosion rate. Also it increases the amount of sulfur in acid gas stream that exits from sweetening and enters to sulfur recovery units. Gas sweetening unit of Bidboland gas refinery (BGR) was simulated with Hysys software. Simulation of BGR had good agreement with industrial data. The parameters such as CS (carbon dioxide in sweet gas), SSRU (the amount of H2S in outlet acid gas stream from stripper to sulfur recovery unit), RAL (rich amine loading) and HPA (reboiler duty per amine circulation rate), were compared for ten blends of DEA (Diethanolamine) and MDEA (Methyl Diethanolamine). According to technical defined parameters, mixed amine with composition of 40 wt. % MDEA and 10 wt. % DEA was a good amine blend for gas sweetening unit in BGR.