Abstract: Four-ball tests were performed to evaluate the on-line self-repairing ability of lubricant with nano-zinc as an additive to the worn steel ball. A mathematical model was established and assessed by a uniform design method, and the impact factors of self-repairing effect were quantified. The contrast test and blank test were designed and conducted to evaluate the self-repairing ability of nano-zinc as an additive. While the morphologies of the worn steel balls were observed on a scanning electron microscopy (SEM). It was found that the experimental result and caculated result were in good agreement, and this suggested the validity of the regression formulae. As an additive, nano-zinc showed the characteristic of self-repairing, i.e., repaired the worn surface of the steel ball, diffused in the contact area, reduced the diameter of worn scar.