Bozüyük, an industrial city attached to Bilecik today, has an important historical past from the foundationof the Ottoman Empire until the years of national struggle. The district, which was located on thehighway route until the railway line was opened, was a center used by the Ottomans as a point ofaccommodation when traveling to the eastern expeditions. The mosque, which was built by Kasım Pashain the first half of the 16th century, was the reason of the existence of Bozüyük city today. The onlybuilding that keeps its authenticity from the complex to the present day is the mosque. The mosque,which has a simple architecture from the outside, has a rich ornamentation program. In this article, thedecorations of the mosque are handled in detail and drawings and photographs related to thedecorations are given.