Recent studies have suggested that statins, the inhibitors for 3-hydroxy-3-methyglutaryl (HMG)-CoA reductase in the mevalonate pathway, exhibit anti-inflammatory effects. However, the immune modulatory effects of statins on the differentiation of CD4+ T cells and their underlying mechanisms are still largely unknown. To address these issues, we examined the effect of simvastatin and inhibitors for protein farnesylation and geranylgeranylation on the differentiation of IL-17-producing T cells (Th17 cells) and Foxp3+ CD4+ T cells. Simvastatin inhibited the differentiation of Th17 cells through the inhibition of HMG-CoA reductase activity but enhanced the differentiation of Foxp3+ CD4+ T cells. Geranylgeranyltransferase I inhibitor, GGTI-298, but not farnesyltransferase inhibitor, FTI-277, mimicked the effects of simvastatin, indicating that the inhibition of protein geranylgeranylation is responsible for the effects. Moreover, Foxp3+ CD4+ T cells developed in the presence of transforming growth factor-β and GGTI-298 functioned as regulatory T cells (Tregs) in in vitro T cell proliferation assay as well as in an autoimmune colitis model. Finally, GGTI-298 induced SOCS3 expression and inhibited IL-6-induced signal transducers and activators of transcription3 phosphorylation in CD4+ T cells. Taken together, these results indicate that protein geranylgeranylation enhances the differentiation of Th17 cells and inhibits the differentiation of Foxp3+ Tregs partly via the inhibition of SOCS3 expression.