Translucent ZrO2 film was successfully prepared by gelling hydrothermally produced nano-ZrO2 powders. The film (300 mu m thick) was found to transmit light to 6.5 mu m (40% transmission) when sintered at 1200 degrees C, but transmission was totally lost after sintering at 1300 degrees C for 1 h. Residual organic material such as urea, which was used for preparing the powder, dominated the transmission of the film in the region between 1.3 and 4.5 mu m when sintered below 1000 degrees C. When sintered above 1000 degrees C, the microstructure controlled the transmission. Both organic residuals and the microstructure of the zirconia were found to determine the transmission in 4.5-6.5 mu m region.