In this work, a method of analysis for directional coupler device is proposed.   Normally, directional coupler operates at lower values of normalized frequency "V_x". So, the corresponding normalized propagation constant "b_x" becomes approximately in logarithm to relation to V_x.  Therefore, the simplified model is used to find the propagation constant, coupling coefficient and their changes with the controlled voltage.  As illustrative applications, the proposed method is applied to the power divider/combiner ratios, the bandpass wavelength filter and three waveguides directional coupler. The analysis of the electro-optically controlled directional coupler becomes simple with more evidence and very good agreement. The effect of the controlled voltage on the coupling coefficient is very weak so, the output power of the directional coupler is affected by the change of propagation constants (i.e. by the controlled voltage).