As though titanium and its alloy have specific tensile strength, excellent corrosion resistance, ect., they have not been widely used until now mainly due to their extremely high processing cost. Therefore, it is considered that the above barrier will become smaller if it is possible to improve the mechanical properties of Ti-alloys by applying surface treatments on properties, especially fatigue strength of about 6kinds of surface treatment Ti-6Al-4V alloys. This is, surface treatments by 2kinds of plasma, 3kinds of DLC (diamond-like carbon) coating and the electroness plating method, have been studied for Ti-6Al-4V alloy . This study presents an analysis of fatigue behavior from the viewpoint of micro-hardness and adhesion work through a new technique in Ti-6Al-4V alloy. In addition, the specimen's surface has been successively observed by the replica method during tests and it seems clear that the fatigue strength is improved by the DLC treatment in comparison with that of the conventional Ti-6Al-4V alloy by about 20% (fatigue limit improvement by 70MPa). Though all the hardness value of these specimens increased, the fatigue limit improved only in the case of two kinds of specimens by DLC coating.