Abstract. This work describes an investigation to reduce positioning error of 3 wheel middle size robot by using a modified odometry system. In this technique the positioning sensor (shaft encoders) are mounted on 3 free-running wheels so the slippage of the driving wheels does not affect the measurements of the sensors. This will result in decreasing the cumulative error of the system. This mechanism accompanying by omnidirectional vision system presents reliable and accurate self-localization method for any 3 wheel driving robot. Experimental results have shown performance improvement up to 86% in orientation error and 80% in position error.