Osteomas are the most common benign bone tumours of paranasal sinuses. In general, they are small and asymptomatic. They may lead to local symptoms such as nasal obstruction, headache and facial pain when they become large, besides more severe signs and symptoms due to orbital and cranial expansion. The treatment in symptomatic and rapidly-growing cases is surgery. In treatment of paranasal sinus osteomas, endoscopic or open surgical techniques, which may be applied according to mass dimensions and localization, are available. Endoscopic endonasal resection is a safe method with successful results and low morbidity. It causes no cosmetic problems. In this study, a giant ethmoidal sinus osteoma invading orbital wall and extending to skull base was reported. It was totally resected by endonasal endoscopic approach.