Pluripotent stem cells have the potential to differentiate into all cell types of the body in vitro through embryoid body formation or in vivo through teratoma formation. In this study, we attempted to generate in vivo neural stem cells (NSCs) differentiated through teratoma formation using Olig2-GFP transgenic embryonic stem cells (ESCs). After 4 to 6 weeks of injection with Olig2-GFP transgenic ESCs, Olig2-GFP(+) NSCs were identified in teratomas formed in immunodeficient mice. Interestingly, 4-week-old teratomas contained higher percentage of Olig2-GFP(+) cells (∼11%) than 6-week-old teratomas (∼3%). These in vivo-derived NSCs expressed common NSC markers (Nestin and Sox2) and differentiated into terminal neuronal and glial lineages. These results suggest that pure NSC populations exhibiting properties similar to those of brain-derived NSCs can be established through teratoma formation.