A study is conducted to improve the suction performance of suction devices which are used to remove polluted air generated by welding or machining process at a spacious working place in industries. Air-curtain is applied around the suction inlet not only to prevent absorbing fresh air from the downstream region but also to improve the suction performance from the upstream region. Four different type air-curtain devices, such as forward, sideward, 45°backward and fully backward, are adopted. Suction effect was investigated depending on the mass flowrate blowing to generate air-curtain. Suction condition is selected to 110,000 Reynolds number which has been widely used on typical suction devices, and a width of blowing passage is chosen to 10% of suction duct diameter. Comparing the computed results, the 45°backward air-curtain method was superior to others. When the blowing mass flowrate was 42% of the suction mass flowrate, the suction region was improved to 38% compared to that of without air-curtain. Keywords : Air-curtain, Suction duct, Suction performance, Bellmouth, Ventilation