Abstract: There are still many problems in Millikan oil drop experiment, such as the large deviation of the measured value of the elementary charge, the "objective function method" to solve the problem of the elementary charge, and the use of ultraviolet lamp on oil drops. If it is not handled well, the teaching effect should be affected. In this paper, the large deviation of the measured value of the elementary charge is considered to be related to the accuracy of the reticle of microscope and the fact that the experimenter does not focuses on the oil drop image. The objective function method is of high precision in solving the elementary charge. The usage is sorted out and the difficulties are clarified. Ultraviolet lamp can be used not only to change the electric quantity of oil drop, but also to directly prove the charge quantization by starting from the oil drop with an elementary charge at the beginning, then measuring the balance voltage after each ionization and using the ratio of voltage.

Key words: Millikan oil drop experiment, elementary charge, objective function, ultraviolet ionization, charge quantization