Placenta specific protein 1 (PLAC1) is thought to be important for murine and human placentation because of its abundant expression in placenta; however, the trophoblast subtypes that express PLAC1 at the fetomaternal interface and the major role of PLAC1 in placentation are still unclear. This study investigated the expression pattern of PLAC1 at the human fetomaternal interface and its involvement in trophoblast syncytialization. Localization of PLAC1 at the fetomaternal interface was studied using in situ hybridization (ISH) and immunohistochemistry (IHC) assays. Real time RT-PCR and Western Blot were employed to exhibit the expression pattern of PLAC1 during human spontaneous syncytialization of term primary cytotrophoblast cells (CTBs). Spontaneous syncytialization of a primary term CTBs model transfected with siRNA specific to PLAC1 was used to investigate the role of PLAC1 during human trophoblast syncytialization. The results showed that PLAC1 was mainly expressed in the human villous syncytiotrophoblast (STB) layer throughout gestation, and the expression level of PLAC1 was significantly elevated during human trophoblast syncytialization. Down-regulation of PLAC1 via specific PLAC1 siRNA transfection attenuated spontaneous syncytialization of primary term CTBs (p<0.05) as indicated by cell fusion index and the expression patterns of the corresponding markers. These data demonstrate the facilitative role of PLAC1 in normal human trophoblast syncytialization.