Abstract: Scuffing appears mainly in sliding contacts under high speed-heavy load conditions. Although scuffing has been investigated for many years,the cause of scuffing has th us far eludedour understanding and as a consequence, its prevention is not always successful,nor its occurencepredictable. Among the factors which have influence on scuffing failure,surface roughness and slidingspeed are very important.So,using a Timken machine,scuffing tests have been conducted onGCr15 bearing steel friction pairs lubricated with pure mineral oil and the effects of surfacetoughness and sliding speed on the scuffing behavior have been studied in detail in this paper.Theresults obtained indicate that the critical load of scuffing decreases with increasing of sliding speed,there is an optimum surface roughness which has highest scuffing resistant,the critical load at firstincreases with decreasing of surface roughness,passes through a maximum and finally decreases atvery smcoth sur face. Using the experimental data obtained in this paper,the validity of severalscuffing medels in literature have been analyzed,the results indicate that none of the models canexplain various scuffing behaviors observed in the tests of this paper.So,based on the idea thatscuffing will occur if plastically deforming asperities are spaced so closely that they coalesce to forma scuffing mark,a new seuffing medel-surface plastic flow has been formulated in this paper.Thismedel hypothesizes that scuffing failure is the results of surface plastic flow. It can explain variousscuffing behaviors observed in the tests of this paper.