Disclosed herein are a rotor assembly for a motor in which separation type rotor cores having the other polarity are disposed between core members of integral type rotor cores having one polarity and the separation type rotor cores and the integral type rotor cores are fixed to each other using wedges or injection molding materials that are formed of a non-magnetic material, such that movement of magnetic fluxes between the integral type rotor cores and the separation type rotor core may be minimized, and a manufacturing method thereof.