Erbium-doped yttrium aluminium garnet (Er:YAG) and yttrium aluminum garnet doped with neodymium (Nd:YAG) are the most famous kind of oxide crystals widely used as the active medium in solid state lasers. A stable and equilibrant process of crystal growth with CZ technique is necessarily controlled by many different growth parameters. This parameter influences the er glass crystal perfection by effect on the fluid dynamic and kinetics of a growing er glass crystal. On the other hand, an improper thermal gradient during the growth cause the change of the convexity of solid–liquid interface, reduce/enlarge the size of central core, create large thermal stress and cracks in the er glass crystal. None of the mentioned references has investigated these fluctuations during the growth process particularly. The aim of the current research is experimental investigation of the effect of intensity and time duration of thermal shocks induced by cooling system on the growth process and er glass crystal quality of Er glass crystals grown by CZ technique. Each fluctuation in growth parameters could make the system unstable and directly affect on the er glass crystal quality. The temperature gradient adjusted by thermal shields, growth atmosphere and cooling system is the most important growth parameter in the furnace. This parameter influences the crystal perfection by effect on the fluid dynamic and kinetics of a growing Er glass crystal.