As to polarimetric radars with vector measurement,the optimal estimation of the target scattering vector under clutter background is studied.Taking the minimized mean square error of the target scattering vector estimation as the criterion,the optimal estimation to the scattering vector based on transmit polarization optimization is formulated.An algorithm is proposed to optimize the transmit polarization based on sequential estimation scheme.Numerical simulation results demonstrate the effectiveness of the proposed algorithm,and show that the performance of transmit polarization optimization is superior to that of the conventional polarization design.