Abstract
The technique of optical pumping that directly and accurately measures number of cold atoms in the magneto-optical trap (MOT) is presented. All the instruments in the technique are not sophisticated and the measurement procedure is simple. The coherent population trapping (CPT) phenomenon in the |5S <sub>1/2</sub> ,F=2>→|5P <sub>3/2</sub> ,F′=2> excitation of <sup>87</sup> Rb atoms is illustrated.