To increase cargo capacity of a liquefied gas carrier intended for marine transportation of liquid ammonia, there are used liquid cargo vessels. In this type of vessels, low-temperature liquefied gases are delivered at a pressure approaching atmospheric. However, the liquid cargo carrier is intended not only for transportation of liquid ammonia to the port of destination. One of its main functions is additional refrigeration of liquid ammonia while being transported, for example, from Trinidad and Tobago to the United States, to its normal boiling temperature. To this end, gas carriers are equipped with additional reliquefaction units. During the passage of the liquid cargo carrier on the specified route, all additional reliquefaction units are under operation. It is shown that during the loading of ammonia in the gas carrier, the pressure in its tanks and the basic physical characteristics of the ammonia change. The reasons for the pressure increase in the tanks are explained and practical recommendations for stabilizing the vapor pressure in them are given.