This article introduces a new blockout device for rotational path removable partial dentures. The concept of rotation axis is discussed, from which a more convenient tool for clinical use is derived. The blockout instrument can be divided into three parts: the acrylic block, the rotation axis and the functional part. In the case of tilted mandibular molars, distal and lingual surfaces of anterior abutments should be blocked out according to the rotational path of insertion. In the blockout procedures, the blockout instrument is mounted between the two rotation centers on the two distal abutments with dental stone, so as to coincide the rotation axis of the instrument and that of the denture to be constructed. After the areas below the survey line of the anterior abutments are aproned with wax, Duralay resin is applied onto the areas above the survey line, and extended to join the functional parts of the blockout instrument. After setting, Duralay resin is removed and the portion above the survey line is trimmed to form a knife edge which corresponds to the survey line. Thereafter, the blockout areas are determined by the rotation movement of the knife-edge shaped Duralay resin along the rotation axis of the blockout instrument. This blockout method is more easily performed compared to the method mentioned by Firtell and Jacobson. In addition to the function of blockout, it can also be assisted in analyzing undercut and diagnosis.