Graphical representation of CP violation effects in neutrinoless double beta decay

We illustrate the graphical method that gives the constraints on the parameters appearing in the neutrino oscillation experiments and the neutrinoless double beta decay. This method is applicable in three and four generations. Though this method is valid for more general cases, we examine explicitly the cases in which the CP violating factors take +/-1 or +/-i in the neutrinoless double beta decay for illustrative clearance. We also discuss some mass matrix models which lead to the above CP violating factors.