Dimensional accuracy is one of the most important factors that govern the accuracy of any impression material, and consequently the accuracy of any impression. The aim of this in-vitro study was to compare the dimensional changes of two impression materials; polyether and addition silicone using both a digital caliper and computerized digital software. A maxillary partially edentulous Kennedy class III with modification 1 acrylic model was used in this experiment. The model had acrylic teeth with missing second premolar and first molar bilaterally. The teeth and the cast was prepared to ensure proper seating of the custom tray fabricated. A dot shaped elevation was prepared on the middle of the occlusal surface of the first premolar and third molar, and on the base of the cast five box shaped reference marks with parallel walls were drilled on the buccal side of the cast, anteriorly and posteriorly, and on both sides.After the cast was prepared, a 2 base plate wax spacer was adapted over the cast, and custom made clear self-cure acrylic resin was fabricated to be adapted to the full extensions of the cast, including the box-shaped reference marks.Two impression materials ; polyether and additional silicone was used in this study. A total of 20 impressions were made, 10 impressions with polyether in the first group and 10 impressions with additional silicone in the second group, in both groups the same tray was used.