Screw loosening is one of the most frequent complications in implant-supported restoration. However, many dental clinicians misunderstand the exact mechanism of screw loosening. Screw loosening can trigger peri-implantitis more severely than what dental clinicians usually think. Just a change of abutment screws cannot solve this complication. The decrease of preload, the repetition of tightening the abutment screw, the effect of friction on the stability of implant-abutment connection, and hard-soft tissue response to strain around the implant-abutment connection are four key words to correctly understanding the screw loosening mechanism. This presentation talks about the cause of screw loosening, theoretical methods to eliminate screw loosening and translation of these methods in clinical situation, mechanical understanding the implant-abutment connection structure, and biological understanding the implant-abutment connection structure.