Background and Aim: The key role of chemokine receptor CXCR4 in the maintenance of stemness property of stem cells has been shown recently. The low level laser irradiation (LLLI) is being used curren tly in a wide variety of clinical cases as a therapeutic tool for wound healing, reli eving pain and destroying tumor cells. The aim of this study was to evaluate the ef fect of LLLI mimicking low level laser therapy (LLLT) on the expression level of CXCR4 gene a few hours after irradiation on human blastocysts. Methods: After the development of human embryos to the firs t grade blastocyst stage, they were irradiated with a low power Ga-Al- As laser at a continuous wavelength of 650 nm and a power output of 30 mW. Total RNA of the irradiated blastocysts and control groups was isolated in grou ps of 1x2 J/cm 2 , 2x2 J/cm 2 , 1x4 J/cm 2 and 2x4 J/cm 2 LLLI. Specific real-time PCR primers were designed to amplify all the two CXCR4 isoforms yet identified. RNA amplifications were done for all groups. Results: We showed for the first time that LLLI makes the h uman blastocysts to increase the expression level of CXCR4 a few hours after irradiation. Moreover, it was shown that two irradiation doses with one day i nterval can cause a significant increase in CXCR4 expression level in human blastocysts. Conclusion: This study revealed that LLLI could be a prolifera tion motivator for embryonic cell divisions through enhanced over-expression of CXCR4 level.