Abstract: In China there are two different opinions on formal translation which is regarded as one of the important hypotheses in Nida's theories. Based on an analysis of the development of formal equivalence in Nida's theories by employing Lakatos's methodology of scientific research programme, the paper explains the cause of the two contradicted viewpoints and points out that Nida stops using absolute formal equivalence and puts it in the framework of functional equivalence by progressively adjusting his theories. And the paper demonstrates the important role of the epistemology of scientific developmental rules in understanding translation theories.