The main object of this research is the game of Othello. We review the history and the playing strategy of Othello and then construct an Othello program. A file structure of its knowledge base was designed and implemented in this work to improve the ability of our program. Although four-by-four Othello was strongly solved, the five-by-five Othello is still not investigated. This study solves five-by-five Othello strongly with personal computer, and furthermore, attempt to extend the solving strategy to six-by-six Othello.