Three phases, phase A (0.04 < delta < 0.08), phase B (delta = 0.14) and phase C (delta = 0.2) appear in CaVO3-delta. The structures of oxygen-deficient CaVO3-delta with delta = 0.14 and 0.20 have been investigated by means of electron microscopy. The electron diffraction study revealed that these compounds have monoclinic supercells with reciprocal unit vectors of a double dagger = g(2), b* = g(1(1) over bar0$)(c)/2 and c* = g(1)/2, where g(1) = g(441)(c)/30 and g(2) = g(<(22)over bar>)7(c)/30 for delta = 0.14, g(1) = g(332)(c)/20 and g(2) = g(<(11)over bar>6)(c)/20 for delta = 0.20, and g(hkl)(c) represents the reciprocal lattice vector of the cubic-perovskite structure. The observed symmetry of the supercells does not agree with any other vacancy-ordered structures found in defect perovskite systems. The observation that the superlattice spots appear most densely in the cubic [110] lattice plane in both compounds suggests that the oxygen vacancies form brownmillerite-type extended defects, namely tunnel defects along the cubic [110] direction.