BACKGROUND Evidence shows thiabendazole has the potential to inhibit angiogenesis in melanoma and fibrosarcoma; however, its effect on oral squamous cell carcinoma has not been previously studied. OBJECTIVE This study sought to assess the cytotoxic effects of thiabendazole on HN5 head and neck squamous carcinoma cell line. METHODS HN5 cell lines were exposed to different concentrations of thiabendazole (prepared from 99% pure powder) for 24, 48 and 72 hours. Cell viability was assessed by the methyl thiazol tetrazolium assay, and IC50 of thiabendazole was calculated. Cells were also exposed to different concentrations of thiabendazole for 48 hours to determine its effect on expression and transcription of vascular endothelial growth factor gene. Expression of vascular endothelial growth factor mRNA was assessed by real-time polymerase chain reaction. The vascular endothelial growth factor release was assessed by the enzyme-linked immunosorbent assay test. RESULTS In all concentrations of thiabendazole except for 200 and 550μM, cell viability was significantly different at different time points (p< 0.05). At 48 and 72 hours, cell viability at all concentrations of thiabendazole (100-650μM) significantly decreased compared to the control group (zero concentration). In addition, cell viability significantly decreased with an increase in thiabendazole concentration. At 48 hours, expression of vascular endothelial growth factor mRNA was significantly lower in presence of 500μM thiabendazole compared to the control group (p< 0.001) and release of vascular endothelial growth factor was inhibited in a dose-dependent manner. CONCLUSIONS Thiabendazole inhibited the proliferation of HN5 cells in a dose-dependent and time-dependent manner. It also inhibited the expression of vascular endothelial growth factor gene.