A 304 stainless steel plate was lap joined to a CP-Ti one by friction stir welding technique. Stainless steel was selected as the top member. Sound dissimilar joints were achieved using an advancing speed of 50mm/min and rotation speeds in the range of 700–1100rpm. A region of vortices of bimetallic weld of 304 stainless steel and CP-Ti was formed in the lap joint fabricated using the highest applied tool rotation speed; this was associated with plasticizing of both members with the aid of a double-shoulder tool. In addition, due to complex material flow, mechanical interlock features were shaped that consists of extruded stainless steel into the plasticized titanium region. A maximum shear strength value of ∼119MPa was achieved; this was found to be close to that of CP-Ti. The lap joint was strengthened by the formation of vortices of bimetallic weld of 304 stainless steel and CP-Ti and mechanical interlock features at joint interface due to complex materials flow.