In a previous study, we synthesized ultrafine particles containing a new ferro-or ferri-magnetic compound in an Y-Fe-O system. This was done by supplying a co-precipitated powder, having an Y:Fe molar ratio of 1 : 1.9, to an argon-oxygen radio frequency (PF) plasma. The resultant new phase had an X-ray diffraction (XRD) pattern not registered in the JCPDS, the standard database of XRD patterns of known chemical compounds. To synthesize this new phase as a pure single-phase product, powder precursors prepared by the amorphous citric gel method and liquid precursors of mixed nitrate solutions with various Y:Fe ratios were supplied to the plasma. It was found that the new phase, which has good crystallinity and some magnetism, is synthesized as an almost pure single-phase product from a powder precursor with an Y:Fe ratio of 1:3.7. By observing the particle-size distributions and shapes (crystal habits) of the particles synthesized from powder and liquid precursors, and referring to previous studies, it was found that further experiments need to be conducted where the feed rates of powder or liquid precursors were varied systematically with all other conditions remaining constant.