We investigated the change of mechanical properties of UP using mixture of SM with VAc or MA that have low glass transition temperature when polymerized instead of SM only for diluents of UP. In case of using mixture of SM/MA, it was elucidated that the toughness of UP was more improved than using SM only. But in case of SM/VAc, it was observed that the content of mixture could not affect on toughness. It was concluded that these results are caused by the effect of each diluents mixtures on stress-relaxation due to phase separation and on decrease glass transition temperature of UP is quite different from each other.