Runout observation in optical disk drive is a common research interest, but very few researches were able to address the runout problem during the track seeking process. This study investigated the possibility of using an on-line observer to estimate the disk runout during track accessing. A model based feedback algorithm for the track-accessing servo is used as the base for the design. The observer in the controller not only helps to provide the necessary feedback information during accessing, but also made it possible to measure the disk runout in the removable media. The experimental results showed that proper tuning of the controller clearly provide the track wobbling information that will make possible further investigation on how to eliminate the runout effect in the accessing process.