Understanding the Zn diffusion behavior in GaSb is very important to accurately control the distribution of Zn during the fabrication of photoelectric devices. The surface concentration and effective surface diffusivity are two key parameters for modeling the Zn profile in GaSb. The experimental results indicated that when the diffusion temperature and time are kept unchanged, the diffusion profiles with pure Zn, Zn-Sb, and Zn-Ga sources differ in the surface concentration and effective surface diffusivity. To quantitatively explain the effects of source composition on the two parameters, the relationship between the two parameters and the vapor pressure surrounding the GaSb wafer in the diffusion process was deduced based on a surface-equilibrium assumption and the substitutional-interstitial mechanism. The Ga/Sb/Zn ternary phase diagram was calculated and discussed for determining the vapor pressures in different source cases. The ratio of surface concentrations and that of effective surface diffusivities between different sources were obtained to quantitatively explain the experiment results. The theoretical and experimental results both indicated that adding Sb to pure Zn source keeps the surface concentration unchanged while slightly decreases the effective surface diffusivity, and that adding Ga to pure Zn source significantly decreases both the surface concentration and effective surface diffusivity.