A solvothermal method to synthesize barium titanate nanoparticles is employed. In this method many raw materials such as oleic acid, surfactant, sodium hydroxide, alkali barium acetate, alcohol, tetrabutyl titanate are used. When the samples are collected, oleic acid which has mush an account will cover the whole particles. To make the samples cleaner, nitric acid is used to transition oleic acid to elaidic acid which is lighter than water then it will float up to be removed. In the progress to wash off the oleic acid the particles were found to reunite unusually: bigger and cubic particles are formed and the shape seems like single crystal but they are reunitions indeed, we used HTEM to prove this. It means self–assembling might happen through the progress, the spontaneous polarization orient, polarity of molecules will affect this progess.