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We present the study on molecular weight dependent sub-Tg surface dynamics of polymer thin films byusing the Nano-step experiment [McGraw et al.Soft Matter 7,7832 (2011)].By varying the molecularweight,we are able to probe the surface dynamics of the free surface below Tg with the polymer sizecomparable to the surface depth.In particular,we define and use a correlation function to comparemeasured and calculated profiles to analyze the transition from the bulk flow to flow restricted to thesurface region.Surprisingly,even for the polymers with Mw=22,000 surface flow is still observed belowthe bulk Tg value.