Ultrafine cobalt-substituted iron oxide particles were prepared by the thermal decomposition and oxidation of the new organometallic precursor, $Co_xFe_{1-x}(N_2H_3COO)_2(N_2H_4)_2$ (x = 0, 0.01, 0.02, 0.03, 0.05, 0.10, 1.00). The organometallic precursors were synthesized by the reaction of Co(II) and Fe(II) ion in a mole ratio of x : 1-x with hydrazinocarboxylic acid, and characterized by quantitative analysis, elemental analysis and infrared spectroscopy. The mechanistic study on the thermal decomposition of the organometallic precursors was performed by TG-DTG and DSC. The cobalt-substituted iron oxide particles were obtained by the heat treatment of the precursors at $350^{circ}C$ and $450^{circ}C$ for six hours in air. The prepared iron oxide was found to have two phases such as ${gamma}-Fe_2O_3$ and a mixture of ${gamma}-Fe_2O_3;and;{alpha}-Fe_2O_3$ at $350^{circ}C$ and $450^{circ}C$ respectively. The particle shape was equiaxial and the particle size was less than 0.05 ${mu}m.$ The coercivity and squareness of the cobalt substituted iron oxide particles increased with increasing cobalt content. Both coercivity and squareness showed higher values at $450^{circ}C.$