Abstract: To study the friction coefficient during particle impact on metal, an effective friction coefficient is defined as a combination of shearing term and ploughing term at each instant. The state variables, which depend on the state of shearing the metallic junctions by sliding or rolling and the state of dragging the material from the front of the particle, are included. In the case of oblique impact of a hardened steel sphere against a mild steel target and continuous sliding of the sphere on the target, the value of shearing term could be near 0.05 only under certain condition of asperity interaction. It was chosen as an adjustable parameter in all of the calculations by Hutchings for consistency with both experiments and calculations. In the case of continuous ploughing, however, the value of ploughing term is a function of the impact process and the initial impact angle and is greater than 0.05 on an average for Hutchings?experiments. Therefore, it is suggested that the variation of sliding, rolling and ploughing state at each instant in the impact process make the coefficient of friction?equal to 0.05 for Hutchings?experiments. In general, the effective friction coefficient during particle impact on metal is far less than the friction coefficient during simple continuous sliding on an average.