Under non-lubricating conditions, the real contact area (Area 1) and its vicinity of micro-gap (Area 2) which influence the frictional characteristics of paper-based wet friction materials, were statistically measured. The effects of the number of running-in cycles (N) and the contact pressure on Area 1 and Area 2 were examined by measuring the reduction in thickness of the friction material and by conducting particle analysis considering the real contact points. The results are as follows: Area 1 increased with N and contact pressure, but hysteresis behavior appeared in the unloading process. However, no hysteresis was observed in the thickness reduction of the friction material. (2) As N increased, Area 1 increased but there was little increase in Areal 2. (3) The average real contact pressure obtained from nominal contact pressure and also the number of contact points (n) based on particle analysis decreased with N, owing to the increase in n. (4) Regardless of contact pressure and N, the relation between Area 1 and n was expressed as a line with a folding point where the slope changed.

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