In the case of internal grinding of a deep hole, the inner surface of the hole was ground by a slender grinding wheel that has a high aspect ratio. A slender grinding wheel is bent by the normal grinding force during the grinding process. Therefore, the form accuracy of the ground workpiece is decreased especially when the aspect ratio of a wheel is high. In our previous study, it was confirmed that the biggest factor that generated the form error of the workpiece was the elastic deformation owning to the normal grinding force. To reduce the form error of the ground workpiece, it is effective to expand the spark-out duration. However, the expansion of the spark-out duration leads to the low productivity. In this study, the form of the grinding wheel was modified by a diamond dresser to improve the form accuracy of the workpiece without expanding the spark-out duration. The modified form of a wheel was determined to compensate the form error of the workpiece generated by the elastic deformation of the grinding wheel. Thorough several grinding experiment, it was confirmed that the form accuracy of the ground workpiece was successfully improved by modifying the form of a slender grinding wheel without expanding the spark-out duration.