In this paper, an extensometer was specifically designed for online deformation monitoring of high temperature pipes and its applicability was verified experimentally. The design procedure and the installation of the extensometer were described in detail. Experiments were carried out to prove the accuracy and the reliability of the extensometer. The laboratory test at room temperature was performed. The reliability of the extensometer at high temperature was validated by on-site monitoring of the deformation of a main steam pipe operating at 540 °C in a power plant. It was demonstrated that the designed extensometer has a high accuracy and an excellent reliability for measurement of the deformation of the high temperature pipes.