Abstract : Boring process is the machining process to enlarge an internal diameter of cylindrical tubes and is considered one of the most important processes in rocket motor manufacturing. In general, vertical boring is used over horizontal boring in order to control dimensional accuracy and concentric of bored tubes. However, vertical boring process required a specific machine which can be a burden to manufacturer. In this study, the lathe boring tool is developed to reduce the cost of acquiring new machine. In a typical boring process using horizontal lathe, the ratio between workpiece internal diameter and length is restricted to 1:3. If the length is too long, defects such as uneven roughness can be observed due to vibration of cutting insert. The new horizontal boring tool is designed based on rotating cutter principle. The designed is simulated using finite element method and tested on S40C cylindrical tubes. The surface finished of tested workpiece is within tolerance and acceptable.