An experimental study on how to reduce the content of singly ionized zinc vacancy (V$^−_{Zn}$ ) in ZnGeP$_2$ crystals to improve their optical quality is presented. Their electron paramagnetic resonance signal has been studied in samples treated in different ways. The data provide a scheme of the reductions that can be obtained by the different methods or by their combinations. The adding of Sn during the synthesis phase helps the production of large‐size single crystals and the reduction of the V$^−_{Zn}$ content. Thermal treatment and electron irradiation are then needed for further reduction. At the fluence of 2 × 10$^{17}$e cm$^{−2}$ , the V$^−_{Zn}$ decrease does not reach a minimum value, so with higher fluences, additional reduction of V$^−_{Zn}$ content is expected.