Abstract: The threshold characteristics of lasers are of great importance to the output of lasers. In this paper, we use a theoretical model, which is based on rate equations, to investigate the threshold characteristics of an end-pumped single-pass rubidium vapor laser using iteration method. The results show that, the influences of temperature and gain medium length on the threshold pump intensity are equivalent, and there exists an optimum combination of temperature and gain medium length which can minimize threshold pump intensity. The influence of pump linewidth on threshold pump intensity is linear, and the influence of central wavelength shift on pump intensity will be smaller if the pump linewidth is broader. Also there exists an optimum Rb atomic absorption linewidth, which can minimize threshold pump intensity. In order to reduce threshold pump intensity, the cell window should be anti-reflectively coated, and the output coupling-efficiency should be less than 80%.