We performed a study to look into the therapeutic application of a novel diagnostic and therapeutic agent, phase change nano droplet (PCND), in combination with 1-MHz ultrasound. It was found using gel phantoms that PCND works as a cavitation accelerator for a 1-MHz ultrasound only when an ultrasound which changes the phase of PCND from liquid to gas (phase change ultrasound) is exposed in advance. A nano droplet that cannot change its phase to a gas does not work even in the presence of the phase change ultrasound. The cavitation induction with the aid of a PCND was observable by B-mode echography as a brightness change. Such a brightness change was also observed in in vivo experiments on tumor baring mice in the presence of a PCND and a phase change ultrasound. Moreover, tissue damage was observed at the site of the brightness change. The lack of either a PCND or a phase change ultrasound did not induce any brightness change, suggesting the same mechanism as a gel phantom works in living tissues. Our results are promising for use in a noble ultrasound therapy system with high selectivity and safety while improving the throughput of current ultrasound tumor treatment systems.