A simple technique for measuring the coupling coefficient and the power transfer efficiency of the optical directional coupler is presented. This technique consists of measuring Fraunhofer diffraction patterns emerging from the output prism coupler that is placed on the parallel waveguides. The measured values for the K(+)-diffused glass waveguide couplers that are fabricated at different diffusion times are demonstrated. A measurement error of <4% for the coupling coefficient is estimated for the 4-6 mm range of the coupling length.