Recent discoveries of the colossal magnetoelectric effect (CME) in TbMnO3 by Kimura and DyMn2O5 by Hur gave new stage on the study of multiferroics. They discovered that electrical polarization is flopped or flipped by magnetic field. Thus, we may control the ferroelectric state not only by electric field but also by magnetic field. Polarization flop by magnetic field and polarization flip by electrical field give four-state of the system, which can be used as memory device. In this paper, recent progress on the studies of magnetic and ferroelectric properties of RMn2O5 is reviewed. When we measure the dielectric and magnetic structure in the H-T phase diagram, we can easily say that the origin of the CME is the phase transition phenomena induced by magnetic field. The same phenomenon was also observed against hydrostatic pressure. We discuss the origin of the polarization of RMn2O5 in terms of magnetic ordering pattern. New results are also shown, in which ferrielectric character and polarization flop were found in RMn2O5.