In this paper, we formulated a four dimensional system to explore the interaction between Tax-expression and CTL immune response in HTLV-1 infected individuals. Two parameters (R₀ and R₁) are derived to study the system dynamics. We analyzed the existence and stability of the disease-free equilibrium, the immune-free equilibrium and the infected equilibrium theoretically. By numerical simulations, the impact of Tax-expression and immune response on the proviral load were demonstrated.