Abstract: A method of treating an aramid monofilament by plasma in such a manner that the monofilament has a wettability by water satisfying the relationship WS.sub.SL .gtoreq.120, W.sub.SL being the reversible solid-liquid adhesion work expressed in mJ/m.sup.2. By way of example, the treatment is carried out by means of a plasma at reduced pressure, and the monofilament is subjected to a coating treatment after the surface treatment. The aramid monofilament is characterized by the following relationships:1.7.ltoreq.Ti.ltoreq.260;40.ltoreq.D.ltoreq.480;T.gtoreq.170-D/3;Mi.gtoreq.2,000; andW.sub.SL .gtoreq.120.In the above relationships, Ti represents linear density in tex, D represents diameter in .mu.m (micrometers), T represents tenacity in cN/tex, and Mi represents initial modulus in cN/tex. The aramid monofilament is particularly useful for reinforcing tires.