Hydrothermal treatment (HT) has been proved as a significant pretreatment in decreasing emissions of NOX pollutants from thermochemical utilization of sewage sludge (SS) derived solid fuel. This study aims to investigate the denitrification of HT and the redistribution of nitrogen (N) in different products so as to speculate the comprehensive pathway of N transformation during hydrothermal process. Results found that only 20% of N remained in hydrochar, whereas the rest of N (nearly 80%) was transformed into other phase. A majority of amino-N in SS was enriched in liquid phase in the form of Org-N at first, then further decomposed to NH4+-N. The remaining amino-N converted to pyrrole-N, pyridine-N and quaternary-N as temperature progresses. Meanwhile, amine-N derived from protein-N formed heterocyclic-N in oil phase via Diels-Alder reaction. NH3, the major nitrogenous gas, was dissolved in liquid as NH4+-N immediately after producing, but increased with prolonged reaction time.