A polymeric nanocapsule encapsulated with 1,1,1,3,3-pentafluorobutane was developed, in which 2,2,3,3,4,4,4-heptafluoro-1-butyl groups were introduced to the polymer terminal for high loading of liquid fluorocarbon. In vitro experiments demonstrate that its liquid/gas-phase transition can be triggered by mild heating. This nanocapsule can be harnessed as a contrast agent for tumor imaging under diagnostic ultrasound.