We have studied the effect of molecular chirality and cell gap on the electro-optic (EO) modulation associated with a homeotropic to twisted-planar (HTP) transition. It is found that an optimum condition of the cell gap and the corresponding chirality exists for the maximum EO modulation. A surface layer of the KTP structure is somewhat thicker than that of a conventional twisted nematic one. The helical pitch predominantly governs waveguiding characteristics and the resultant EO properties of the HTP structure. The polar and azimuthal surface anchoring on treated homeotropic alignment layers is also discussed.