This paper discusses quantification method of proficiency of sport skill with Iaido as a subject. Iaido is a modern Japanese martial art using a Japanese sword. A player executes a sequence of actions: drawing the sword from scabbard, cutting a virtual opponent, removing blood from the blade, and sheathing the sword. We measured motions of athlete playing the Iaido by motion capture system, then analyzed motion trajectory to predict the skill level as a score. The predicted scores were compared with the professional rating scores to evaluate the validity of the subjective assessment of professional. The comparing result shows the validity and fairness of subjective assessment of professional. Also, the results shows the validity of our proposed rating algorithm.