The method of minimum overall cost is one of the optional methods that are used for techno-economic analysis about the many kinds of equipment renewal modes.However,it is a controversial issue on how to handle the old equipment’s cash realizable value in the decision-making renewal,when calculating the overall cost. In this paper,the overall cost’s calculation formula was deduced rigorously. Furthermore,it pointed out that the old equipment’s cash realizable value should be counted while calculating the overall cost of such schemes as the old equipment’s continued using,heavy repair and modernized refit,however,it should not be considered when calculating the overall cost of prototype replacement or technological replacement.