The essence of natural deduction lies in that any assumptions can be introduced during our proofs. After summarizing some distinguishing features of different Natural Deduction Systems, this paper provides a teaching case on natural deduction, in which there are more deductive rules with assumptions. Furthermore, by making use of the serial numbers of proof sequences, this paper encodes and keeps track of their thinking and strategies of the demonstrators. This paper argues that the said approach, which is more advantageous for the students to grasp the logical methods and proof strategies, is helpful to stimulate their independent thinking and develop their own proofs, and consequently to improve their logical thinking ability and abilities on proving meta-theorems.