HatemMokhtarErrabti¹,MoustafaAbdouElsyad², Aisha ZakariaHashem Mustafa ³. ¹B.D.S, Colleague of Master Degree, Faculty of Dentistry, Mansoura University, Egypt. ²asso.Professor of Removable Prosthodontics, Faculty of Dentistry, Mansoura University, Egypt. ³Assistant Prof. of removable prosthodontics, Faculty of Dentistry, Mansoura University, Egypt. Abstract: Objectives:this in vitro study was aimed to evaluate and compare base deformation of mandibular over dentures retained by either ball or locator implant attachments. Methods:acrylic model with silicone mucosal simulation was fabrication and two laboratory  implant were inserted in the canines region. Two duplication experimental over dentures were fabricated to be used with either ball and locator attachment .3pairs strain gauges were attached over denture lingual polished surface right (ch1.2.3) loading side and left side (ch6.7.8) non loading side. To measure the strain around the attachment Tow strain gauge were attached at the midline (ch4.5) to measure strain at the midline of the over denture.  50N vertical static load was applied unilaterally and bilaterally atthe 1st molar area through a universal testing device to measure the strain using a multi-channel digital strain meter. Health Criteria (USPHS criteria). Results:during unilateral loading ball attachment recorded higher tensile strain at loading side (ch1.2.3) than locator attachment but locator attachment recorded higher strain at non loading side (ch6.7.8). During bilateral loading locator attachment recorded higher compressive strain than ball at the majority of channels. During unilateral lodging،ball attachment showed highest strain at ch2 on loading side and lowest strain at ch8 on the loading side. For locator attachment the highest strain was recorded ch5.for ball attachment unilateral loading recorded higher strain than bilateral loading at (ch1.2.3) loading side while bilateral loading recorded higher strain than loading at gh (ch6.7.8) non loading .for locator attachment, bilateral loading recorded higher strain than unilateral at the majority of channels. Conclusions:significant mandibular denture base deformation over implant is present with ball attachment compared to locator attachment .reinforcement of denture base over the implants may be recommended when ball attachments are used for implant retained overdenture. Keywords:Denture base deformation, Mandibular complete overdenture, and Implant assisted complete overdenture.