A discussion is made on the thermodynamic equilibrium of nitrogen cycle in the sea. Most of the NH4+, NO2- and NO3-; must be converted to NO3- at thermodynamic equilibrium state. The seasonal variations of nitrogen compounds in English channel, Long Island Sound the Yellow Sea and the East China Sea suggest that the NH4+ and NO2- are not converted sufficiently to NO3-. The system of NH4+, NO2-and NO3- are not in the thermodynamic equilibrium state, but there is a difference in the degree of conversion. The degree of conversion is low in the Yellow Sea and the East China Sea. The system of NH4+, NO2- and NO3-, is not in the thermodynamic equilibrium state, mainly due to activity of phytoplankton. The difference in the degree of conversion in NH4+ to NO3- is related to period of growth of phytoplankton. The NH4+ is converted to NO3- more sufficiently in the state of long period, as in the English Channel and Iong Island Sound. The conversion is not sufficiently in the state of short period, as in the Yellow Sea and the East China Sea.