Abstract
The neutrinoless double beta decay is an important process to test the nature of neutrinos. If neutrinos are Majorana particles, the decay can be observed in experiment and the lepton number is not conserved. We derive some formulae of the parameter of this process called the effective Majorana mass and use new fit data of neutrino oscillations to give constraints on it. With the assumption that all neutrinos have similar mass, we conclude that the two mass hierarchies would give different values and the CP phases have significant effect.