Peripheral CD4 + CD8 + T cells have been identified as a T cell subset existing in animals and humans. However, the characterization of CD4 + CD8 + T cells, their relationship with T memory (T M ), T effector (T E ), Th1/Th2, Treg and Th-17, remain unclear. This study was to characterize the CD4 + CD8 + T cells. The results from human subjects showed that activated T cells were CD4 + CD8 + T cells, comprised CD4 hi CD8 lo , CD4 hi CD8 hi and CD4 lo CD8 hi subsets. They expressed CD62L hi/lo , granzyme B (GrB), CD25, Foxp3, interleukin 17 (IL-17) and the cytokines of both Th1 and Th2, and had cytolytic function. These findings suggested that CD4 + CD8 + T cells had over-lap function while they kept diversity, and that T cells could be divided into two major populations: activated and inactivated. Hence, the hypotheses of Th1/Th2, Treg and Th-17 might reflect the positive/negative feedback regulation of immune system. When compared to GrB + CD62L lo T effector (T E ) cells, GrB + CD62L hi T central memory effector (T CME ) cells had a quicker response to virus without CD62L loss.