Adhesive forces dominating the flowability of the segregation-free iron based powder consist of liquid bridge force, electrostatic force and van der Waals force. They were calculated and the absorption isotherm of water vapor on the segregation-free iron based powder was measured. The result indicated that the van der Waals force between binder on iron powder and lubricant powder and that between lubricant powders are main forces that determine the flowability of segregation-free iron based powder in the relative humidity less than 91%.