The effect of operating conditions on the removal efficiency of deposited particles using highspeed air jet was studied with particular attention to the humidity of surrounding air during particle deposition and air-jet removal. The experimental results showed that the removal efficiency of particles deposited in a low humidity atmosphere, decreased with increasing drying time after the deposition. In the case of a deposition under high humidity, the removal efficiency increased at the beginning of drying and decreased rapidly after reaching the maximum efficiency. For drying over 80 hrs, however, the humidity of the surrounding air during the particle deposition didn't affect the removal efficiency.The efficiency also depended on the humidity during the air-jet removal, and reached the maximum value at 67% relative humidity. Such a variation of the removal efficiency has been well explained by the change of Hamaker's constant with the adsorbed layer-thickness of water molecules and the variation of liquid surface tension with the thickness of liquid bridge.