Ordered domains due to oxygen vacancies in oxygen-deficient Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.7}$ single crystals have been imaged by dark-field electron micriscopy using the diffuse diffraction spots centered at (\textonehalf{},0,0). When projected along the $c$ axis, the ordered domains have longitudinal and transverse dimensions of \ensuremath{\sim}200 and \ensuremath{\sim}20 \AA{}, respectively. The longitudinal direction of the domains is parallel to the Cu-O chain direction (i.e., $b$ axis). The vacancy-ordered domains are estimated to occupy \ensuremath{\lesssim}3% of the sample volume.