A new technique has been developed to locate the interface shape in the floating zone of lithium niobate materials. The floating zone is formed and uniformly heated by a CO2 laser with an axisymmetric ring-shaped beam generated by an axicon optical system. The emitted thermal radiation from the hot object has been measured by an imaging radiometer. The interference of the CO2 laser with the measurement of the radiometer is avoided by setting the spectral bandpass of radiation to 3-5 mu m. The location of the gas-solid or gas-melt interface is determined through the significant difference of the emitted radiation between the solid or melt and the background. Since the lithium niobate is viewed as a semitransparent material and the emittances of the solid and melt are significantly different, the locations of the melt-solid and gas-melt-solid interfaces are determined according to the change of the emitted radiation intensity. The floating zone is resolidified, and the interior of the resultant solid is observed by SEM (scanning electron microscopy) to identify the location and shape of the melt-solid interface. The results predicted by the variation of radiation intensity are consistent with those observed from the SEM.