Abstract: The coated fuel particle (CP) is one of the most important components of fuel element for high-temperature gas-cooled reactor (HTR). It is very important to precisely measure the size of coated particle online because it is a key parameter and the density of each coating layer can be obtained throughout the measurement results of size and quality of coated particle. Various methods, including X-ray micrographyprojection method, Vtype groove method, photo-electronic length measuring method, metallography method, image analysis method, computer automated optical microscopy and particle size analyzer method are discussed. With the need of high precision and speed on-line measurement for coated particles, computer automated optical microscopy and particle size analyzer method will be applied in the R&D of HTR coated particles widely.