Optimization of the biphenile chloromethylation process with paraphormaldegide has been done in the presence of ZnCl2 with HCI gas by the Box-Wilson method of mathematical planning of experiment. The 4,4`-(dichloromethyl)-biphenile yield dependence on the biphenile-paraphormaldegide ratio, temperature and reaction duration has been studied. A mathematical model of the process has been developed and optimal conditions for the biphenile chloromethylation procedure has been determined.