Under the strong pressure from the United States, the Legislative Yuan of R.O.C. has passed the Pharmaceutical Affairs Act article 40-2(2)(3)(4) which was promulgated on February 5, 2005 by the president and brought R.O.C. into an era of data exclusivity. Data exclusivity was created by the United States, by whom this system was forced through all over the world by means of trade negotiations and international agreements. The protection object of data exclusivity is the test data of prescription drugs. This thesis concludes that the legal principle of data exclusivity is the protection of the trade secrets residing in the test data of prescription drugs from unfair competitions from other drug companies, and owing to the financial incapability of generic companies to undergo the safety and effectiveness test by themselves, what data exclusivity actually results is that the owner of the original test data acquires a period of non-competition market exclusivity, and hence gets an opportunity to profit from their investment to the test data. TRIPs § 39.3 provides the least protection standard for test data of prescription drugs. It demands members who requires as a condition of approving the marketing of pharmaceutical products which utilize new chemical entities that first applied in this member country, must protect the submitted test or other data which is undisclosed, the origination of which involves a considerable effort, against unfair commercial use. This thesis construes “unfair commercial use” as acts using or referring to the test data that bring commercial benefits for other companies without providing any opportunity for the owner of the original test data to recoup from his investment to the test data. Thus, data exclusivity system excels the TRIPs standard. However, the “unfair commercial use” language of TRIPs § 39.3 only indicates some form of protection against naked exploitation of the test data is required. TRIPs does not encompass a data exclusivity obligation for member countries.