The crystal structure, oxidation state, electronic properties of unusual niobates which contain niobium clusters condensed low dimensionally, were investigated by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), electrical resistivity measurements, and Raman spectroscopy. The results of XPS experiment indicate that there are three niobium oxidation states (Nb4+, Nb3+, Nb2+) in Ba2Nb5O9 and these oxidation states are expected to be stable considering their atomic coordinates. Pure BaNbO3 phase was synthesized and its niobium oxidation state was examined in order to compare with those of niobium in Ba2Nb5O9. Electrical resistivity measurements on Ba2Nb5O9 show negative dρ/dT behavior between 160 K and 600 K. Similar behavior was observed in yttrium substituted niobates, Ba2-xYxNb5O9. The possibility of relationship between low dimensionality and anomalous properties of these niobates is discussed.