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The magnetite superparamagnetic nanoparticles were synthesized by using a co-precipitation method. The surfaces of magnetite nanoparticles were modified by carboxylic acid group of poly (3-thiophenacetic acid). The crystalline structure and the composition of core magnetite nanoparticles were not changed after surface coating with a very thin outer shell of carboxylic-acid group. The magnetite and the surface-coated magnetite particles were well monodispersed. The diameters of the nanoparticles were about 10.4 nm and 12 nm, respectively, determined by using transmission electron microscopy and dynamic light scattering method. The superparamagnetic behaviors were examined by measuring magnetization hysteresis loops and zero-field cooling and field cooling magnetizations as a function of temperature from 5 K to 300 K. The blocking temperatures of the magnetite and the surface coated magnetite were around 245 K and 220 K, respectively. The superparamagnetic behaviors of the magnetite and the surface coated magnetite nanoparticles were discussed in the context of the change of magnetizations with temperature.