Abstract: Effects of low-frequency Ar,N_2 RF-plasma on wettability of medical stainless steel were studied in details. Wettability of EVAL solution, the morphology and bonding strength of EVAL coating on the stainless steel with and without plasma pretreatment were investigated. The correlation of surface wettability with surface free energy and surface structure was also established. Wettability was measured by contact angles of water droplet on the samples. Surface free energy (dispersive force and polar force) was calculated by contact angles of liquids whose surface tensions were known. Wettability of medical stainless steel after plasma pretreatment significantly increases and the optimum conditions of treatment are: N_2 gas plasma, bias voltage 100 V, time 10 min. Moreover, the uniformity, density and the bonding strength of EVAL coating on the stainless steel pretreated under the optimum condition are remarkably improved. Contact angle measurement results show that mainly because of the contribution of polar forces, the surface free energies of samples after plasma treatment are enhanced. ATR-FTIR, AFM, XPS results indicate that the modification of wettability and the increasing of surface free ~energy are due to surface cleaning, surface etching and surface activity.