Various wet friction materials, in which different fillers are compounded on the surface of fibrillated fiber base, were prepared. The friction properties of these materials were pursued at various slip speeds, loads and oil temperatures. Streibeck Diagrams of the data were obtained, and the effect of fillers on the frictional characteristics was investigated. The following results were obtained; (1) The slope of the Streibeck Diagram should be positive to have smooth frictional characteristics. (2) Abrasive powders effectively improved the slip characteristics of friction materials. (3) The effect of the abrasive fillers was further improved by using less compliant paper.