The Patent Act provides a patentee has the opportunity to amend the description, claim(s) or drawing(s) of a granted patent. There are many important issues about a post-grant amendment of patent, including what is the extent of a post-grant amendment of patent, which is involved with how to balance rights and interests between the patentee and the third party, and how may a post-grant amendment of patent affect civil suits and administrative litigation. All issues in this thesis are compared with the Japanese law, including how may the conception of the integrity of patent affect a post-grant amendment of patent, and the interpretation and legislative suggestions of the requirements for a post-grant amendment of patent, and what should the civil court do when patentee filing a request for amending the description, claim(s) or drawing(s) of a granted patent during civil procedure, and whether a post-grant amendment of patent could be used to initiate a rehearing action to request a review of the final and binding civil judgment, and whether patentee can file a request for amending the description, claim(s) or drawing(s) of a granted patent during an administrative litigation procedure.