Nanostructured hollow spheres of Co3O4 were synthesized by ultrasonic spray pyrolysis. Nanocrystals forming the structure of spheres differ in size and packing density, depending on the synthesis conditions. The samples were certified by X-ray diffraction, scanning electron microscopy, low-temperature nitrogen sorption, vacuum pycnometry, Raman and XPS spectroscopy. The field and temperature dependences of the magnetization showed that the size and packing density of nanocrystals determine the magnetic characteristics and the Néel temperature of hollow Co3O4 spheres.