A new method for the fabrication of Fe3O4 nanoparticles enveloped by polymeric nanocapsules is proposed. This method is characterized by combining a double emulsification with the interfacial coprecipitation of iron salts to form Fe3O4/polymer composite nanocapsules in a single step. To demonstrate the viability of this approach, methoxy poly(ethylene glycol)-poly-(lactide) (MePLEG) was chosen as the shell material for Fe3O4/MePLEG nanocapsules. In addition to the versatility offered for fabricating nanocapsules with different shell materials, the method was found to be convenient for adjusting the magnetite content of the nano capsules from 0 to 43%. In addition to their confirmed T-2-weighted magnetic resonance imaging (MRI) enhancement, the than MePLEG nanocapsules in an acoustic investigation. Furthermore, the low resultant composite nanocapsules display much more obvious acoustic responses toxicity of these composite nanocapsules, as confirmed by our study, combined with their magnetic and acoustic properties ensure that these composite nanocapsules have great potential in acting as ultrasonic/MRI dual contrast agents.