  We propose a novel scheme to efficiently polarize and manipulate the electron
spin in a quantum dot. This scheme is based on the spin-orbit interaction and
it possesses following advantages: (1) The direction and the strength of the
spin polarization is well controllable and manipulatable by simply varying the
bias or the gate voltage. (2) The spin polarization is quite large even with a
weak spin-orbit interaction. (3) Both electron-electron interaction and
multi-energy levels do not weaken but strengthen the spin polarization. (4) It
has the short spin flip time. (5) The device is free of a magnetic field or a
ferromagnetic material. (6) It can be easily realized with present technology.
