Purpose: To evaluate the influence of the attachment type and inter-implant distance on the retention of 2-implant retained mandibular overdenture during 12 months of simulated function. Material and Methods: Three transparent acrylic resin casts simulating a completely edentulous mandible were fabricated. The retentive forces (N) of two implant-retained mandibular overdentures were estimated by a universal testing machine. Six groups were computed on three casts; two ball attachments with inter-implant distances (IIDs) 23mm were placed in the canine, 31mm were placed in the premolar and 40mm were placed in the first molar regions respectively and two magnetic attachments were placed on the same 3 IIDs.Mean value and differences between and within groups were calculated were performed using a one-way ANOVA, repeated measure ANOVA, and independent student's t-test,
P ≤ 0.05. Results: Significant differences were detected between ball attachments at all IIDs after 12 months of simulated function, P ≤ 0.05, but no significant differences with magnetic attachments, P > 0.05. Statistically significant differences were observed between the ball and magnetic attachments at different IIDs, P = 0.001. The correlation between the attachment type, IIDs and time for both ball and magnetic attachments were significant, P=0.001. Conclusions: The retention of the ball was more than the magnetic attachment. Inter-implant distance influences the retention of both the ball and magnetic attachments. The retention of ball attachment has been influenced by 12 months of simulated function.