The optical-grade diamond film was prepared by DC plasma jet CVD method, and a double-sided polishing was made by EDM and mechanical polishing technology. The investigations of the internal stress at different temperature on the infrared transmission were tested in detail. SEM and AFM were used to study the polished surface morphology, surface roughness was 0.12nm and 0.25nm. XRD and Raman were characterized on the diamond films, the results showed a higher intrinsic quality. The internal stress distribution and infrared transmittance were investigated at 25°C and 100°C temperature respectively. The infrared transmission mechanism of diamond film was also discussed. The main reason of decreasing of infrared transmittance was the increasing grain collisions, which caused more scattering and absorption of incident light with temperature increasing.