Cr clusters have been produced by a plasma-gas-condensation type cluster deposition apparatus, and studied using a time-of-flight mass spectrometer and a transmission electron microscope. The Cr clusters formed in high pressure inert (Ar and/or He) gas atmosphere are of an A15-type structure. When an O₂ gas is mixed with the inert gases in the source (sputtering) chamber, a bcc phase is formed together with Cr₂O₃. The O₂ gas introduction leads to an increase in the gas temperature of the source chamber probably due to release of the formation enthalpy of the oxide. The A15 phase is annealed by such excess heat and becomes the equilibrium bcc phase. The sizes of bcc clusters are smaller than those of the A15-clusters, probably due to the heterogeneous nucleation promoted by the oxide formation.