Objectives:This study was done to compare between two designs of four implants used for assisting mandibular complete overdentures regarding the peri-implant alveolar bone height changes. Methods:Ten healthy male completely edentulous patients of age ranging from 50 to 60 years were selected for this study. All patients received conventional complete dentures. The patients were divided randomly into two equal groups according to the four-implants design concept. Group A: Received four axially placed implants in canine and second premolar areas and Group B: Received four implants (two axial implants inserted in canine areas and two 30 degree distally inclined implants inserted in the first premolar areas). Ball attachments were screwed into implants to retain the overdentures. Peri-implant alveolar bone height was evaluated immediately, 6 months and 12 months after insertion of definitive overdenture. This was done using standardized periapical radiographs. Results:When comparing between the means of peri-implant vertical bone loss (VBL) during the first and second 6months in each group (anterior and posterior), a statistically insignificant difference was found, although the higher VBL during the first 6 months. When comparing between the means of VBL around the anterior and posterior implants of group (A) during the observation times of the study, a statistically insignificant difference was found, in spite of the increased resorption around the posterior implants. Also, the VBL around inclined posterior implants of group (B) was found to be insignificantly more than the VBL around vertical anterior implants through the first and second 6 months. Finally, the mean of VBL around implants in group (A) was statistically insignificant less than that around implants in group (B). Conclusions:Within the standard level of peri-implant bone resorption, the four axially inserted implants and the four implants inserted according to the All-on-four design can be used for assisting the mandibular complete overdentures. Assisting the mandibular complete overdentures by four axially inserted implants (in canines and second premolars areas) can be considered slightly more advantageous than the four implants inserted according to the All-on-four design.  Recommendations:Other studies with long follow up time and more patients in addition to other methods of evaluation are recommended to differentiate between the two studied designs.