The rearing of seabass (Lates calcarifer ) seeds at the Mariculture Centre Batam, Indonesia use a recirculation system with shock temperature of the water in fish tank . Otherside , Viral Nervous Necrosis (VNN) virus disease is one of the deadly infections in mariculture, including sea bass. During the VNN outbreak, o n June-July 2023, there were mass mortality of seabass seed in the hatchery, within 2-8 cm in size. Clinical symptoms: fish swim weakly, some swim in circles, body color becomes dark accompanied by scratchy wounds on the fins, lack of appetite. Treatment of vitamin C, multivitamins , antibiotics added o nto pellets not reduced the mortality . The water temperature of the fish tank using the recirculation system was originally set at 32-33oC . The next treatment was done aim to reduce the mortality due to VNN disease. T he water temperature was set till 38oC using the heater in reservoir tank , the hot temperature kept. There was a significant decreased in the mortality within a 2-3 days. L aboratory test for infected fish result ed still positive for VNN using PCR test and confirmed with histopathology test. Next, the water temperature was set again at 32-33o C, nevertheless mass mortality occurred again. So the water temperature was set again at 38o C. Mortality was decrease drastically. Within a week , minimum, the seeds slowly health and normally back in two weeks there was no mass mortality with t he survival rate gained 75%. Hot water temperature shock treatment in resirculation system makes fish metabolism increase, increase immunity , nat ural selection for sicked fished, weaken the pathogens and had effective to decrease the mortality due to VNN outbreak.