In this paper, knotted suture threads for microsurgery are evaluated from the view point of tension and elongation by using a developed testing machine. The suture threads for microsurgery are thin and easy to rupture. That makes a suture difficult. If a surgeon pulls the thread too strong when he or she makes a knot, it will rupture. On the other hand, if a surgeon pulls it too weak, it will become untied. Therefore, we want to know the mechanical properties of the thread with a knot. That will be a reference when a surgeon handle the thread or when a microsurgery support robot is designed. While a surgeon makes a knot, there are two chances to rupture the thread; when he or she forms the first throw or the second throw. To measure the mechanical properties of the first throw, an initial throw of a surgeon's knot is used in this paper. Also, to measure the mechanical properties of the second throw, a square knot is used. The measurement of the thread 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. Tension control is implemented to pull the thread from both sides that is similar to the operation which surgeons do. The result shows a thread with a square knot is weaker than the first throw of a surgeon's knot.