High quality InSb p-n junction is formed by Cd diffusion using a two temperature zone technique. The junction has a breakdown voltage of 14.4 V at 77 K, exceeding those of the state of art. I-V characteristics as a function of temperature are carefully studied. The reverse breakdown voltage of the p-n junction increases as the temperature is raised, indicating that the mechanism responsible for the breakdown is an avalanche, rather than a tunneling induced process. RA product of the diode is measured to be 1.2×106 Ω·cm2 at zero bias and remains fairly constant up to a reverse bias of 4 V.