Purpose: To compare the effect of linear versus quadrilateral distribution of four implants retained mandibular single overdenture using ball attachments on the retention of mandibular overdenture and on peri-implant outcome. Materials and Methods: Twelve patients were selected for this study with average age from 40-60 .They had maxillary dentulous arches opposing mandibular edentulous arches. Maxillary occlusal plane rehabilitation was made .Mandibular single denture was constructed and duplicated in clear acrylic resin to be used as surgical stent. According to the distribution of implants, the patients were randomly divided into two groups; Group I where the four implants were distributed in a linear form in the interforaminal distance, two in the site of mandibular lateral incisor, and two in the mandibular canine site, group II where the implants were distributed in quadrilateral distribution in the canine, second premolar region. After osseointegration period, ball attachments were used to retain the mandibular single overdenture. The retention of the mandibular overdenture and the peri-implant outcome were evaluated using the following variables: (1) modified plaque index; (2) modified bleeding index; (3) probing depth; (4) implant mobility using the Periotest instrument. The measurements were evaluated at time of mandibular single overdenture insertion (T0), six month (T1), and twelve month (T2) after mandibular overdenture insertion.   Results: The descriptive statistics of retention showed statistically insignificant difference in the retention measurements at the three periods of time within the same group (p= 0.493 in group I and p= 0.255 in group II). However, there was  statistically significant difference between the two studied groups at each observation time (p < 0.001at the three observation times). The comparison of periimplant outcome showed statistically insignificant difference between two groups and statistically significant difference within each group, except implant mobility as it was zero mobility in both groups and along all the period of the study. Conclusion: Quadrilateral distribution of four implants used to retain mandibular single denture is more advantageous than linear distribution, however further radiographic evaluation, and study the effect of using different attachments are recommended.