Haemorrhagic Septicaemia (H.S.) is an economically important disease in veterinary field and vaccination forms an important option in order to reduce the mortality and thereby economic losses to farmer. Since many authors have reported instability of H.S. vaccine, the development of thermostable vaccine is today's urgent need. More recently, Deuterium Oxide (D2O) has been proposed as a thermostabilizer in some vaccines like polio vaccine. Therefore, a study was carried out to know the thermostabilizing property of D2O in H.S. vaccine. Inactivated alum precipitated H.S. vaccine in H2O and 70% D2O were prepared. Vaccines after passing the sterility, safety and potency test were used in stability studies. The thermostabilization studies of H2O and D2O vaccine stored at 4°C, at 24 ± 2°C and at 40°C was carried out by potency test in mice. At 4°C, D2O and H2O vaccines maintained their original potency upto 45 days, whereas loss in potency was observed by 0.09 and 0.15 protection units in H2O and D2O vaccines respectively in 90 day. At 24±2°C, D2O and H2O vaccines lost their potency by 0.4 and 0.25 protection units respectively in 45 days, whereas loss in potency was by 0.78 and 0.5 protection units in D2O and H2O vaccines respectively in 90 days. At 40°C, D2O and H2O vaccines lost their potency by 0.8 and 0.5 protection units respectively in 45 days. Whereas, fall in potency was observed by 1.62 and 1.2 protection units in D2O and H2O vaccines respectively in 90 days. The results of present studies of thermostabilization showed that, 70% D2O incorporated vaccine lost potency at greater rate than H2O vaccine.