Electrolyte gating with ionic liquid has attracted significant attention as a powerful tool for greatly modulating the physical properties in a variety of materials.However,for electrolyte gating in transitional metal oxides,there has been controversy on the origin of electronic phase transition.(See Ref.[1-3] and many other papers published in the past three years)Herein,we report the control and the mechanism of metal-insulator transition via ionic liquid gating in hole doped manganite films.A resistance increase of more than four orders of magnitude was obtained.The oxygen vacancies and their distribution in the gated manganites were observed.We reveal that the metal-insulator transition is caused by the oxygen vacancies originated from the electrochemical reaction between the trace water inside ionic liquids and the manganite films,and the water is an indispensable factor in this process.These findings thus pave a new way to manipulate the physical properties and to utilize the functionalities based on the electrochemical phenomena for oxide materials.