To measure the accurate position of mobile robot, dead-reckoning method using the encoder attached to each wheel is conventionally used, since it is easy to compute and inexpensive to develop. However, that method is useless when slip occurs and error is accumulated with time. This paper proposes a position estimation method using encoder trailer composed of 2 encoders only(ET-2). This method provides accurate position information even when slip occurs, and can reduce accumulated error if we select the proper link lengths of encoder trailer. Experimental results show the performance of ET-2 when slip occurs. Finally, accumulated systematic error from encoder resolution is analyzed in comparison with an existing method with encoder trailer composed of 3 encoders. (As a matter of convenience, we will call the existing encoder trailer ‘ET-3’)