Abstract: The instrument of Millikan oil-drop method is simple, the method is. ingenious, and the result obtained is reliable. It is the model of experimental physics. However, it is not ideal in dealing with data of this experiment by a computer. The method of finding radius of oil-drop anda new method and programme of finding the greatest common divisor of Q (K) under the condition of errors are given in this paper. The train of thought in this method is clear and it is convinced. It has basically solved the problem of finding the greatest commen divisor of Q (K) within the limits of certain errors.