Function exhaustion of specific cytotoxic CD8+ T cell in chronic virus infection partly results from the low levels of CD4 help, but the mechanisms by which CD4 help T cell required to control hepatitis B virus infection are not well understood. In this study, we investigated the role of interleukin-21-producing CD4+ T cell response in viral control of hepatitis B virus infection. HBcAg-specific interleukin-21-producing CD4+ T cells in blood were detected in patients with hepatitis B virus infection. Patients with acute hepatitis B had greater HBcAg-specific interleukin-21-producing CD4+ T cells in blood compared with chronic hepatitis B patients, and there was no statistical significance between immune active chronic hepatitis B patients and inactive healthy carrier patients for these cells, whereas frequencies of these cells negatively correlated with HBV DNA levels but positively correlated with HBc18-27-specific IFN-γ-producing CD8+ T cells. Moreover, interleukin-21 sustained HBc18-27-specific CD8+ T cells in vitro, and interleukin-21 production by HBcAg-specific IL-21-producing CD4+ T cells of acute hepatitis B patients enhanced IFN-γ and perforin expression by CD8+ T cells from chronic hepatitis B patients. Our results demonstrate that HBcAg-specific interleukin-21-producing CD4+ T cell responses might contribute to viral control by sustaining CD8+ T cell antiviral function.