Disc brake system with the dual diameter opposed-piston caliper design has differently sized pistons in front and in the rear. The system adopts a power transfer system in which the disc brake rotor is pressed by the smaller sized piston at the entrance of the disc brake rotor intake and by the more powerful larger sized piston at the back. This prevents the wobbling motion of the disc brake rotor and reduces noises significantly. The 4-piston caliper also allows a wider contact area for the disc brake rotor while keeping the size compact, and the efficient location of pads is adjusted as necessary to stabilize the pressure.