Abstract The decomposition behavior of phenylalanine, as a model compound of algal protein, in water at high temperature was investigated in a quartz mini-batch reactor. The conversion of phenylalanine at 130–190°C as well as the decomposition pathways and nitrogen transition behavior in the hydrothermal process at 220–340°C with a batch holding time of 5–240 min were determined. The results showed that the conversion of phenylalanine is extremely low at 130–190°C, and that provided a reference for extracting high value-added protein during hydrothermal liquefaction of algae. The major product at 220–280°C is phenylethylamine; however, the yield of styrene is increased with the increase of reaction temperature and holding time. In water at high temperature, phenylethylamine is obtained via decarboxylation of phenylalanine, while styrene is produced via deamination of phenylethylamine under higher temperature and longer holding time; phenylethanol is further formed via the hydration of styrene. Most of nitrogen in phenylalanine is firstly transferred into phenylethylamine via the decarboxylation of phenylalanine, and then further transferred into water-soluble NH4+ via the deamination of phenylethylamine.