Abstract: Largesized freestanding transparent diamond films of 50mm diameter and 300μm thickness were prepared by microwave plasma chemical vapor deposition (MWPCVD). The growth rate of the diamond film was only 12μm/h when the diamond film was grown at a methane concentration of 2%, and the infrared (IR) transmittance reached 70% in the range of 5004000cm-1 after the film was polished on both sides. A high growth rate of 78μm/h was achieved for the film grown at a methane concentration of 4%. The thickness of the film was 260μm after it was polished on both sides and its IR transmittance in the range of 5004000cm-1 reached about 60%. Meanwhile, the IR transmittance was almost the same in the central and fringe regions. These results imply a promising application of largesized thick diamond films in IR windows.