Interferon induced transmembrane protein-1 (IFITM1) is one of transmembrane protein which is differentially expressed in uterus during estrus cycle and pregnancy, that IFITM1 gene is highly expressed in estrus stage by the effect of estrogen, and in parturition by the effect of PGF2 alpha. This genes are also up-regulated in cells with hyperactivation of the WNT/β-catenin signaling pathway. In this study, to identify a function of IFITM1, the binding partner of IFITM1 were determined using immunoprecipitation and LC- MASSMASS methods. 1, 3 and 5 ug of polyclonal anti-IFITM1 antisera were used for immunoprecipitation, and the 75 kDa of specific band was detected in silver stained polyacylamide gel. This band were chracterized using LC-MASS-MASS, and revealed this band is glucose regulated protein 75 (GRP75) which binds to p53 and inhibits the p53 action in nucleus. To identify the localization of GRP75 in cells, immunocytochemical approach has been applied, and GRP75 is expressed in mitochondria of L929 murine connective tissue cells. Co-localization study between IFITM1 and GRP75 in L929 cell identified that these two proteins were closely expressed in mitochondria. Although the role of the interaction of these two protein need to be clarified in various biological phenomena, this data suggest that close interaction of IFITM1 and GRP75 may regulate cellular functions in uterus on sets of estrus cycle and pregnancy.