Background: Trismus is a restricted mouth opening due to the tonic contraction of muscles of mastication. It may occur mostly in HNF cancer patients undergoing radiation therapy treatment. This study was undertaken with the purpose of finding out the prevalence of trismus in patients undergoing radiation therapy. Method: 89 patients with HNF cancer patients receiving radiation therapy were included in the study. Inter incisal distance was measured using sliding digital vernier calliper. Patients with ≥ 35mm mouth opening were considered as trismus cases. Result: Out of 89 patients included 72 were diagnosed as trismus; hence the prevalence was 81%. Conclusion: Prevalence of Trismus is very high ( 81 %) in patients undergoing radiotherapy secondary to HNF cancer. Early diagnosis of trismus in these patients can help in time management and also planning of preventive strategies.