of physics，to reveal that magnitude of any charged bodies takes separation value，and determine magnitude of

the electron’s change. Suitable choices for oil-drop will help us to improve the measuring accuracy. Some books about

physical experiment provided choice rules，however it is not explained the reasons why we shall make that

choices. In this paper，we focus on choice rules for the oil-drop experiment，and reveal the physical reasons by

combining theoretical and error analysis. These results presented in this paper will provide theoretical support for

choice rules of oil-drop.