The polarization degradation of ferroelectrics, reflected in the hysteresis loop as a decrease of remnant polarization, is a serious concern in applications. In situ observation of the polarization and domain structure evolution are carried out for -Oriented 0.74Pb(Mg1/3Nb2/3)O3-0.26PbTiO3 (PMN-26PT) single crystal under cyclic electric field. Measurements of the polarization hysteresis loops are conducted under a polarized light microscope (PLM), which is effective in detecting the domain structure of the single crystal. Prior to the polarization measurements, the crystal was poled. Upon cyclic electric field, reversal of monoclinic (MB) takes place, and the phase transition from MB to orthorhombic (O) phase occurs under positive field, but not under negative field of an electric cycle at a field magnitude up to 840 V/mm. The polarization loop is asymmetrical with the positive coercive field (+Ec) much less than the negative one (−Ec). As the field cycles, micro-cracking occurs preferably in  direction, and a sudden drop appears in the curves of polarization versus number of cycles. Upon electric cycling at a field magnitude of 840 V/mm, the negative polarization increases slightly with the cycle number, indicating the rotation of a MB phase toward the applied field direction.