Abstract
In RoboCup middle size league, the official rules have been changing year by year. In 2008, goal's colors (blue, yellow) changes no color goal such as human soccer goal. Furthermore, the ball changes orange ball with FIFA official balls. Therefore, in the middle size league, it has being very difficult to recognize the objects. In this research, we developed ball recognition algorithms, because most teams are using a method of orange color extraction for recognizing ball now. This paper shows experimental results of two ball recognition algorithms. These algorithms are Hough transformation method, and ball estimation method by green inversion.