The specimens of two base metals and material friction-welded with hetrogeneous steels of SM40C and STS304 have been prepared and the characteristics of rupture and crack propagation of them have been examined. In the friction-welded material, the width of HAZ is 3.0mm for STS304 and 3.4mm for SM40C. The hardness distribution in HAZ of SM40C is decreased gradually as being getting off interface, but that in HAZ of STS304 is decreased remarkedly and the value of hardness becomes a little lower than that of the base metal in region of 1mm from interface and becomes a little higher than that of the base metal. The tensile strength of the friction-welded material appeared a little lower than that of the base metal and rupture by tensile load is developed in HAZ of STS304 and the position of rupture is at region of low hardness(1mm from interface). The crack propagation rates(da/dN) in both HAZ of the friction-welded material are a little higher than those in both base metals, but the difference are less except for the case of SM45C in low .DELTA. K value. It has been ascertained that the crack in the interface propagates selectively along the HAZ of SM40C and consequently the crack propagation rate is almost similar to that in the HAZ of SM40C.