We propose Polarization-Shift-Keying (POLSK) phasediversity homodyne system insensitive both to polarization state and phase noise in coherent optical communications. The proposed system can reduce the influence of state-ofpolarization (SOP) fluctuation on the receiver sensitivity by using the multiplying detection with in-phase and quadrature optical signal components of two orthogonal polarizations. Furthermore, the proposed system is insensitive to phase noise similar to other polarization modulation systems and is attractive in high speed transmissions since the receiver is homodyne type. The most attractive features of the proposed system are that neither a polarization controller nor electronic feedback loop for matching the SOPs is needed and that two problems, polarization state and phase noise, are solved. We theoretically analyze the receiver sensitivity considering of the elliptical SOP of received optical signals and the power penalty of the proposed system in the case of the unbalanced optical 90° hybrid.