This paper is to clarify the relationship between keyword and subject word and why and how to index keyword. It is demonstrated that the keywords are the key terms or short phrases based on or picked out from the title, the abstract and the main text, and can reveal the core content of the paper. It is concluded that the keywords are not covering the main content of the paper but also using terms from the Medical Subject Headings list from Idex Medicus whenever possible. The key terms and short phrases exactly delineated the theme of the paper are at first choice and three to ten of them are perferable.