Narrow-band radiation thermometers with center wavelengths near 0.65 μm are frequently used as standard thermometers at high temperatures. The long-term stability of ten Topcon 0.65-μm radiation thermometers was assessed at NMIJ by using fixed-point blackbodies and spectral responsivity measurements. Most of the changes are due to shifts in the center wavelength of the interference filters to longer wavelengths. Even when the center wavelengths shifted, the filter widths and transmittances remained quite stable for some radiation thermometers, but one was found for which the bandwidth increased from 15.7 to 17.2 nm and the transmittance decreased by 6%. Three Barr filters were found to be very stable in wavelength. The output signals of 0.65-μm Topcon radiation thermometers were within 2% • year−1 without correcting for the wavelength change and within 0.2% • year−1 after the correction. Keeping the objective lens clean is very important for radiation thermometers. Large output decreases were observed in early 2000 for many radiation thermometers at NMIJ. The output changes were as large as 1% and were recovered by cleaning the objective lens.