Arsenic poisoning in ground water of Gangetic zone of Bihar causes lots of health issues to human. LD50 is a primary screening step for the evaluation of the toxic properties of a compound. The objective of present work to calculate the oral LD50 and MPD of sodium arsenite in mice. The present research, different doses like 10, 15, 18, 20, 25 and 30mg/kg body weight of Sodium arsenite was given orally by gavage method to the six different groups of Mus musculus. Signs and symptoms of toxicity and probable death of mice were observed on 1st day and then on 2nd, 3’d, 4th and 5th days to calculate the Median lethal dose (LD5O) and maximum permissible dose (MPD) of Sodium arsenite. The LD50 was found to 18 mg/kg body weight after oral administration in Swiss albino mice. Finally, MPD of Sodium arsenite by oral route in mice was found to be 2 mg/kg body weight.