The crystal structure of antiferroelectric Pb(Yb1/2Nb1/2)O3 has been characterized. Both x-ray diffraction and transmission electron diffraction show two sets of superlattice reflections originating respectively from the B-site atom ordering and the antiparallel lead-atom displacements which suffer within the orthorhombic a(o)-b(o) plane along the a(o) direction. The room-temperature crystal symmetry is considered to be orthorhombic with space group Pbnm (D2h16) and lattice parameters a(o) = 5.918 angstrom, b(o) = 23.453 angstrom and c(o) = 8.221 angstrom. High-resolution electron microscopy confirms the diffraction results. A structural phase transition from the lower-symmetry phase to the cubic paraelectric phase occurs at about 302-degrees-C on heating.