Purpose:To compare the accuracy of different impression techniques and different impression materials on the passive fit of milled bar to support mandibular overdenture using All-On-Four implant concept. Materials and methods: U-shaped mandibular edentulous model was constructed from heat cured acrylic resin. Soft liner material used to simulate oral mucosa. Clear acrylic guide template fabricated for implant placement. Four dummy implants (3.7 mm X 13mm and 4.2mm X16mm) were placed in interforaminals region according to the “All-on-Four" concept. The anterior implants placed vertically, while the posterior implants were placed with 30⁰.  Closed tray (snaps on)at abutment level, and open trays (long transfer coping) at implant level  were used with two impression material (Polyether (PE)  and polyvinyl siloxane (PVS). Ten impressions were taken for each technique using medium bodied consistencies. Subsequently, the impressions were poured with type IV die stone.Milled bar was fabricated on twenty working cast with cantilevered portion (10mm), then casted with cobalt chromium alloy. The frameworks were screwed onto the master cast and the vertical misfits were measured by an optical microscope. Results: Statically there was no significant difference in implant position (anterior and posterior) with impression materials.  It can be seen that the higher mean vertical gap distance was found in long pin transfer copying than snap on techniques in PE (14.36 ±3.7 versus 8.5 ± 4.4 respectively). Statically there was a high significant deference between long pin transfer coping and snap on using PE impression material (p < 0.001). While there was no significant difference between long pin transfer coping and snap on using PVS impression material.  Conclusion: there was significant difference between polyether and poly vinyl siloxan, in the long pin transfer coping with snap on technique. The limitation of this study included that the vertical gap distance measured occurred with impressions and their casts were made under very strict laboratory conditions Keywords:Polyether, Polyvinyl siloxane, vertical gap, Milled bar, all-on Four Concept.