The effect of cysteamine added to maturation medium on nuclear maturation, ATP content and glutathione (GSH) content of porcine oocytes was investigated. The nuclear maturation up to the metaphase stage of second meiotic division (the M-II stage) of cumulus cell-enclosed oocytes (COs) matured with various concentrations (0, 15, 45, 150, and 450 µM) of cysteamine was observed. The maturation rate of the 150 µM group was significantly higher (P < 0.05) than that of the control (0 µM) group. Although the ATP content of oocytes derived from COs matured with 150 µM cysteamine was the same as that of the immature group, the ATP content of COs matured without cysteamine was significantly higher (P < 0.05) than that of the immature group. Conversely, the GSH content of oocytes derived from COs matured with 150 µM cysteamine was significantly higher (P < 0.05) than that of the immature group. However, the content of the group matured without cysteamine was not significantly different from that of the immature group. These results indicate that cysteamine added to maturation medium may stimulate the nuclear maturation of porcine oocytes by increasing the GSH content without enhancing the oocyte ATP content.