The multiferroic material YbMnO3 has been reported to exhibit bothferroelectric and antiferromagnetic orders in the ground state. Of these twoorders, the ferroelectric order is associated with theP63/mmc-to-P63cm structural transition, which occursaround 1270 K. The interesting feature of the ferroelectric state is that acloverleaf domain structure with a pseudo-six-fold symmetry is observed intransmission electron microscopy images with the beam incidence parallel to thehexagonal axis. To understand the origin of the formation of the cloverleafdomain structure, we have examined the crystallographic features of theferroelectric state in YbMnO3 by transmission electron microscopy. Inthis study, particularly, we adopted the experimental condition that electronbeam incidences are perpendicular to the hexagonal axis. It was, as a result,found that there existed various ferroelectric domain structures including thecloverleaf domain structure under the present condition. The notable feature ofdomain structures found in this study is that each domain structure basicallyconsists of six domains, whose domain boundaries are terminated at one point.Because this feature makes us reminiscent of a discommensurate structure in anincommensurate state, we took high-resolution electron micrographs of areasincluding domain boundaries. Their analysis indicated that a domain boundarycould be identified as a discommensuration with a phase slip of π/3.It is thus understood that the cloverleaf domain structure should be one ofdomain morphologies for a discommensurate structure, which is related to thebreak of the translational symmetry.