A directional coupler composed of two vertical multiple-slotted silicon wires with trapezoidal cross-section is characterized using a full-vectorial finite element method in terms of the electric fields, where the perfectly matched layer absorbing boundary conditions are adopted. The effects of the structural or material parameters including the sidewall angle, the gap between the coupled waveguides, and the width and index profiles in the slot regions on the behaviors of the directional coupler are taken into account. The effective indexes of the even and odd modes and the corresponding coupling length, both in quasi-TE and quasi-TM modes, are presented. The strongly-hybrid nature of the guided-modes is effectively demonstrated. The results show that polarization-independent directional couplers can be realized by properly choosing the structural and material parameters.