Abstract: This paper aims to analyze errors of the high accuracy ultrasonic thickness measurement of thin walled carbon steel pipe in one nuclear power plant. Through theoretical analysis, test verification and site verification, it is verified that the main reasons of error are due to the pipe inner surface roughness, the probe damping and wave attenuation pulse cycles. The principle for selection of high accuracy thickness measuring probe for thin walled carbon steel pipe with rough inner surface is recommended and the requirement for specimen surface roughness is introduced.