Environmentally　persistent　free　radicals　(EPFRs)　have　attracted　increasing　research　interest　in　recent　years.　Herein,　the　generation　of　EPFRs　during　the　hydrothermal　carbonization　of　sewage　sludge　(SS)　was　studied.　First,　the　surface　morphology,　functional　groups,　constituent　elements　and　free　radicals　were　characterized　for　a　holistic　description　of　the　raw　SS　and　the　selected　hydrochar　obtained　from　hydrothermal　carbonization　of　SS　(SHC).　Then,　the　impact　of　hydrothermal　temperature,　residence　time　and　initial　pH　on　the　formation　of　EPFRs　was　explored　in　detail　through　the　investigation　of　g-factors　and　intensities　of　EPFRs　identified　in　SHC.　The　results　have　shown　that　the　formation　of　EPFRs　was　affected　by　the　factors　mentioned　above,　in　which　the　impact　of　temperature　is　the　greatest.　Two　types　of　EPFRs　were　spotted　in　the　hydrochar,　oxygen-centered　(O-centered)　and　carbon-centered　(C-centered)　EPFRs,　which　were　caught　in　120-150　degrees　C　and　260-280　degrees　C,　respectively.　Moreover,　the　intensities　of　Electron　Paramagnetic　Resonance　(EPR)　signals　enhanced　with　increasing　hydrothermal　temperature.　Whereas,　residence　time　and　initial　pH　only　affected　the　amount　of　EPFRs　in　a　manner.　Additionally,　the　half-life　of　the　O-centered　EPFRs　and　the　C-centered　EPFRs　was　determined　as　long　as　160.45　days　and　401.10　days,　respectively,　indicating　that　EPFRs　are　stable　in　a　long　time.