Needlelike Sr_0.5Ba_0.5Nb₂O₆ powder particles were prepared from SrCO₃, BaCO₃, and Nb₂O₅ in the presence of a mixture of molten SrCl₂ and BaCl₂. When the composition of the mixture of oxide and carbonate was fixed to be SrCO₃:BaCO₃:Nb₂O₅ = 1:1:2, the phase and morphology of the product particles were dependent on the composition of molten salt. The exchange reaction, BaO + SrCl₂ = BaCl₂ + SrO, enriched Sr in the oxide phase. The relation between the compositions of the salt mixture and the reaction product was determined, and needlelike Sr_0.5Ba_0.5Nb₂O₆ particles were obtained by using the molten salt containing 18 mol % SrCl₂.