[en] The pair annihilation spectrum of free negatons and positons in relativistic optically thin plasma is numerically calculated. The relativistic Maxwell-Boltzmann distribution of electrons has been assumed. We approximate the resulting spectrum by an analytical function of temperature and frequency. The annihilation rate and the emissivity energy emitted per unit volume per second are given by one-dimensional integrals which were computed independently of the more complex radiation spectrum calculations. An analytical formula for the spectrum in the nonrelativistic limit is given. (author)