This paper reports static and dynamic control of the phase separation structures of polymer blended nanoparticles. The phase separated nanoparticles were prepared from various combinations of polymers by evaporating good solvents from the polymer blended solutions after adding poor solvents into the solutions. Transmission electron microscope observation of the nanoparticles shows that two types of phase separation structures were formed in them. One is Janus type structure and another is core-shell type structure. When the difference of the Flory-Huggins solubility parameter (δ) between two blended polymers is large, one polymer having a higher δ value encapsulated another polymer. As a result, a core-shell type structure is formed. On the other hand, when the difference of δ value is small, both moieties of polymer are exposed to the poor solvent, and then, a Janus structure is formed. Moreover, when the thermoresponsive polymer was blended with a hydrophobic polymer, a Janus type or core-shell structure was formed by changing the preparation temperature. The core-shell type structure was transformed to a Janus type structure by heating the nanoparticles. This control and transformation of structures of nanoparticles can be applied to nano-sensing devices, switching devices and drug delivery systems.