Growth differentiation factor 8 (GDF8) is a member of the transforming growth factor-β that has been identified as a strong physiological regulator. The purpose of this study is to investigate the effects of GDF8 on porcine oocytes during in vitro maturation (IVM). We investigated a specific gene transcription levels in oocytes and cumulus cells (CC) after IVM by realtime PCR arry, and specific protein expression and activation levels in matured CCs by western blotting. Each concentration (0, 1, 10, and 100 ng/ml) of GDF8 was added in maturation medium (TCM199) during process of IVM. Data were analyzed by ANOVA followed by Duncan using SPSS (Statistical Package for Social Science). Data are presented as the mean and Differences were considered significant at P < 0.05. After 44 h of IVM, oocytes are mechanically denuded from CCs with 0.1% of hyaluronidase, and then the separated oocytes and CCs were sampled following each group. To assess the effect of GDF8 on specific gene transcription level changes as a dose response during IVM, the realtime PCR array was performed. In CCs the 1- and 10 ng/ml of GDF8 supplement group showed the transcription co-factors CBP and SP1, cell metabolic regulator MAPK1, and cumulus expansion related genes Has2, Cox-2, Ptx3 and Areg transcription levels were significantly distinguished with control when hierarchically clustered by Euclidean distance with average linkage method after IVM. In matured oocytes the 10- and 100 ng/ml of GDF8 supplement group showed the maternal factors JMJD3 and Zar1, transcriptional regulator FOXO1, Sirt1 and Sirt2, mitochondrial activity factor Sirt3, ACSL3 and ACADL, anti-apoptosis gene BCL-2, and oocyte secrete factor BMP15 mRNA transcription levels were significantly distinguished compared with control. To determine effect of GDF8 supplement during IVM, the GDF8 down steam canonical regulator SMAD2/3 protein phosphorylation levels analyzed in CCs by western blotting. The 10- and 100 ng/ml supplement groups showed significantly increase phosphorylated (P)-SMAD3 (1.56 and 1.34 times higher than control) protein levels (P < 0.05). In conclusion, supplement of GDF8 during IVM activates FOXO homolog transcription and induced cumulus cells expansion via activation of SMAD3 signaling in CCs. While process of IVM, the transcriptional landscape changes in CCs may consequently result maternal factors accumulation and mitochondrial activation in oocytes.