Venting for a diesel tank is an essential requirement to keep the tank stable by breathing in during suction & breathing out the excess pressure created inside because of vaporization. Vent valves with inappropriate specs affects the rate of supply of fuel during vacuum built up of pressure resulting in tank explosion or causes damage to the user during filling. Capacity of the tank, ambient temperature, environment, Return line temperature and application are the factors contribute in finalizing the venting specification. Considering these factors, study has been done on a metal & plastic fuel tank for a commercial vehicle application. Venting specification has been finalized for both plastic & metal tanks analytically. Vent valves developed and validated to meet the functional & durability requirements.