Arrest in embryo development during in vitro culture has been reported in various mammals. Although some cause of the arrest have been suggested, little is known of the way that can overcome the arrest using in vitro culture system. The antioxidant, $eta$-mercaptoethanol($eta$-ME), has been shown to play an important role in embryo development. This study was designed to examine the effect of $eta$-ME on the developing boving embryos produced in vitro by IVM and IVF. To select a, pp.opriate concentration of $eta$-ME during whole culture period (7 days), various concentrations (10, 50 and 100$mu$M) of $eta$-ME were added to the CZB medium and their effects was significantly higher in 100$mu$M of $eta$-ME. The effects on development of embryos cultured with and without somatic cells to blastocyst stage were greater in FCS treatment (56.6 and 29.3%) than in BSA treatment(25.5 and 12.8%). We also evaluated the effects of $eta$-ME addition on the blastocyst formation when embryos at different stages were exposed to 100$mu$M $eta$-ME. $eta$-ME promoted increased development of embryo to blastocyst stage and the effect was greater in 6-cell to morula embryos than in embryos fewer than 4-cells at the initiation of treatment. The results suggested that $eta$-ME can improve bovine embryo development by overcoming the arrest in early development.