Recently, some US patents are invalidated by Korean Intellectual Property Office or Courts. Sanopi Aventis case(2009) can be listed as one of those cases at issue. In this case, the interpretation of the "Transition Phase of Claim" was debated at the court. US Patent filed through PCT(Patent Cooperation Treaty) to Korea was invalidated by the Korean Supreme Court based upon the Korean Patent Law Art. 42(written description requirement). Patent Court held valid that "consisting essentially of" as a transition phase under the context of Korean Patent Law Art. 42 as is the same case in US. However, Korean Supreme Court`s understanding was somewhat different. "Consisting essentially of" is actually too vague for the persons` ordinary skilled in the art to interpret the exact meaning of the claim in the biotechnology area and considering the prosecution history of this claim at issue, Korean patent becomes more comprehensive that the US one in the course of national filing to Korea through PCT. In addition, the wording of Korean Patent Law Art. 42 is different from that of US. Specific feature of this case was main reasons of this Supreme Court case. While, after this decision practitioners in Korea will not use "Consisting essentially of" type of transition any more due to the risks of being invalidated. KIPO has to present a more detailed guideline for clearing the misunderstanding regarding the transition phase.