Mobile robots have a large scale use in industry, military operations, exploration and other applications where human intervention is risky. When a mobile robot has to move in small and narrow spaces and to avoid obstacles, mobility is one of its main issues. An omni-directional drive mechanism is very attractive because it guarantees a very good mobility in such cases. Also, the accurate estimation of the position is a key component for the successful operation for most of autonomous mobile robots. In this work, some odometry aspects of an omni-directional robot are presented and a simple odometer solution is proposed.