We study the Bose-Einstein condensation (BEC) for a relativistic ideal gas of bosons. In the framework of canonical thermal field theory, we analyze the role of particles and anti-particles in the determination of BEC transition temperature. At the BEC transition point we obtain two universal curves, i.e. valid for any mass value: the scaled critical temperature as a function of the scaled charge density of the Bose system, and the density ratio of anti-particles versus the scaled critical temperature. Moreover, we numerically investigate charge densities and condensed fraction ranging from the non-relativistic to the ultra-relativistic temperature, where analytical results are obtained.