Objective:To investigate the expression characteristics of phosphorylated enhancer of Zeste homolog 2(EZH2)in head and neck squamous cell carcinoma(HNSCC)and their effect on chemosensitivity.Methods:Fifty-three patients with HNSCC treated between January 2018 and March 2021 in Tianjin Medical University Cancer Institute&Hospital were selected.Expression levels of p-EZH2S21,p-STAT3Y705,HIF-1α,and Ki-67 in HNSCC tissues were analyzed by immunohistochemical staining.Western blot was used to detect p-EZH2S21 expression in HNSCC tissues and cell lines.HNSCC cell lines stably transfected with wild type(EZH2-WT)or S21 mutant EZH2(EZH2-S21A)were construc-ted.CCK8 and colony formation assays were performed to detect the effect of EZH2 and S21 phosphorylation on HNSCC cell proliferation and their sensitivity to cisplatin(DDP)and 5-fluorouracil(5-FU).Results:Elevated levels of p-EZH2S21 were observed in HNSCC tissues and positively correlated with p-STAT3Y705(P<0.05)and HIF-1α(P<0.01)levels.p-EZH2S21 level correlated positively with lymph node metastasis(P<0.000 5),T(P<0.05),N(P<0.000 1)and AJCC stages(P<0.05).In vitro experiments confirmed that EZH2 expression promoted cell prolifer-ation and attenuated chemosensitivity of HNSCC cells.Inhibition of p-EZH2S21 restored HNSCC cell sensitivity to DDP and 5-FU.Conclusions:p-EZH2S21 plays an important role in tumor progression in HNSCC,and phosphorylation at S21 is an important way for EZH2 to affect HNSCC cell proliferation and chemotherapy sensitivity.