The dynamic responses of a main-chain liquid crystalline polymer (LCP) during the polycondensation reaction under electric fields have been studied for the first time by a modified thin-film polymerization technique using a polarizing microscope. The evolution of the in situ liquid crystal (LC) texture during polymerization under electric fields is found to be quite different from that without electric fields. Compared to the low-molecular-weight LC systems, the polymerized thermotropic poly(p-oxybenzoate/2,6-oxynaphthoate) (P(OBA/ONA)) system does not show distinct morphological change from the fluctuating Williams domains to the dynamic scattering mode. The dynamic response of the LC phase during the polymerization is strongly dependent on the frequency and strength of the external field. In a low-frequency range, molecular orientation is found to be in the low-voltage range. In a medium-frequency range, electric field induced flow (electrohydrodynamic instability) is observed. In a high-frequency range, LC molecular directors cannot respond fast enough, so the system appears to be stationary in a relatively wide voltage range. The changes in inherent properties of the reaction system also greatly affect the response of the LC phase to the electric field; thus, the electrohydrodynamic instability changes with the reaction time accordingly. The low- and high-frequency bounds of the electrohydrodynamic flow regime decrease with an increase in reaction time.