In this paper, the effect of sterilization on a thin suture thread is evaluated, and verification of a developed testing machine is conducted. The suture threads for microsurgery is thin and easy to rupture. That will lead a medical accident, so that the evaluation of the thread's performance is important. One of the factors which decides the performance of the thread is sterilization. The thread must be sterilized for being utilized in a medical operation, and generally it deteriorates the performance of the thread. Tension and elongation of sterilized threads and non-sterilized threads are measured, and the difference among them is discussed. The measurement of the threads is conducted by using a developed testing machine. Reaction force observer (RFOB) is implemented in the machine to measure tension and elongation of the thread at the same time. Also, tension control is implemented to pull the thread from both sides that is similar to the operation which surgeons do. The validity of the testing machine is confirmed through the measurement.