MgO doped lithium niobate (MgLiNbO3) is a relatively hard crystal and is difficult for dry etching. Dry etching rate and morphology control of MgLiNbO3 are key technologies in fabricating optoelectronic devices based on lithium niobate. Etching characteristics of MgLiNbO3 crystal are studied by using Plasmalab System 100 (Oxford Instruments) with mixture gases of SF6/Ar. The etching rates of different working parameters including inductively coupled plasma (ICP) power, reactive ion etching (RIE) power, working pressure and SF6/Ar flow ratio are evaluated. The surface profile is also affected by various ratios of SF6/Ar gas mixture. The optimal etching conditions for MgLiNbO3 ridge-shaped waveguide are found to be ICP power of 1000 W, RIE power of 150 W, total gas flux of 52 mL/min (standard condition of 0 ℃, 1 atm), chamber pressure of 0.532 Pa and the gas volume ratio of SF6/(Ar+SF6) of 0.077. Optical ridge-shaped waveguide with approximately 2.5 μm depth, 74.8° etching sidewalls and smooth top surface is successfully fabricated using the optimized etching conditions.