A new series of metal-substituted ϵ-iron oxides, ϵ-Ru Fe O (x=0 (1), 0.005 (2), and 0.014 (3)) nanoparticles (average size=20 nm) is synthesized by sintering iron oxide hydroxide with ruthenium hydroxide in a silica matrix. The samples are characterized by inductively coupled plasma mass spectrometry, and energy dispersive X-ray spectroscopy (EDS) of transmission electron microscope. The X-ray diffraction pattern shows that ϵ-Ru Fe O has an orthorhombic crystal structure with a space group of Pna2 and the Rietveld analyses show that Ru is doped selectively at the regular octahedral C site among the four non-equivalent Fe sites (A, B, C, and D site). Magnetization vs. temperature plots show that the Curie temperature (T ) depends on x, and T =498 K for 1, 497 K for 2, and 496 K for 3. Magnetization vs. external magnetic field plots indicate that the coercive field (H ) increases from 17.7 kOe (1) to 20.3 kOe (3). This increment of 15 % on H is attributed to the single ion anisotropy of the magnetic spin on Ru (4d , S=1/2).