Mitochondrial dysfunction is found in oocytes and transmitted to the offspring due to maternal obesity. This is curable by endoplasmic reticulum (ER) stress inhibitors such as salubrinal (SAL). Recently pigs are considered as a model animal for biomedical research due to its physiological similarity with human. Pig oocytes have shown ER stress mostly in metaphase II stage. ER stress is hindering the in vitro embryo production (IVP). This study investigated the effect of ER stress inhibition by using SAL during 44 h of in vitro maturation (IVM) of oocytes at 1, 10, 50 and 100 nM concentrations. Firstly, we defined the concentration of SAL during IVM of pig oocytes. SAL at 10 nM showed higher (44.2 to 55.6%, P<P0.05) development competence to the blastocyst state than control and other concentrations after parthenogenetic activation (PA). Secondly, we sorted out the time-dependent treatment at 10 nM of SAL for IVM of oocytes. It revealed that treatment with SAL during 22 to 44 h and 0 to 44 h of IVM improved PA embryonic development significantly (40.5, 51.7 and 60.2% for control, 22 to 44 h and 0 to 44 h of IVM, respectively, P<0.05). Glutathione (GSH) level is an indicator of cytoplasmic maturation of oocytes. Reactive oxygen species (ROS) have a harmful effect on development competence of oocytes. For this, we determined the intraoocyte levels of GSH and ROS after 44 h of IVM. It was found that SAL increased intraoocyte GSH level and also decrease ROS level (P<0.05). Finally, we performed somatic cell nuclear transfer (SCNT) after treating oocytes with 10 nM SAL during IVM. SAL treatment significantly improved blastocyst formation of SCNT embryos compared to control (24.7 vs. 39.6%, P<0.05). Our results indicate that treatment of pig oocytes with ER stress inhibitor SAL during IVM improves preimplantation development cloned pig embryos by influencing cytoplasmic maturation in terms of increased GSH content and decreased ROS level in IVM pig oocytes.