Primary oocytes that are arrested in first meiotic prophase for years enter maturation process to meet a critical precondition for successful fertilization. During maturation, oocyte finishes meiosis I and progresses to the metaphase II stage, achieving meiotic maturity. Although importance of oocyte maturation for oocyte quality has been recognized, it is not fully understood for molecular mechanisms underlying oocyte maturation. Here, we found that dexamethasone-induced Ras-related protein 1 (RASD1), a member of RAS superfamily of small GTPases, was expressed in the mouse ovary. Immunohistochemical analysis revealed that Rasd1 expression was dominant in oocyte cytoplasm. Real-time PCR and RT-PCR analyses showed that Rasd1 mRNA was steadily expressed in germinal vesicle (GV), germinal vesicle break down (GVBD), metaphase I (MI) oocytes, but decreased in metaphase II(MII) oocytes during oocyte maturation. Konckdown of Rasd1 using RNAi system in the GV oocytes suppressed oocyte maturation through disruption of meiotic spindle and formation of misarranged chromosomes. Taken together, Rasd1 is a critical factor for MI-MII transition of oocyte and is involved in the regulation of spindle formation during oocyte maturation. Further study is needed to examine relationship between Rasd1 and spindle formation in MI-MII transition.