Abstract: Micro-size carbon fibers (MCFs) with a diameter of 0.4-0.6 μm and a length of tens of micrometers were prepared on a coal-based anode by dc arc discharge in a mixture of helium and acetylene. The adsorption of cadmium ions by the MCFs in solution was studied. The effect of the functional groups, time, pH value and initial concentration of solution on the adsorption of cadmium ions was investigated. Results showed that the acid oxidation resulted in an increase in the number of oxygen-containing groups. The adsorption capacity of the oxidized MCFs increased significantly. The pH value has a great influence on the adsorption of cadmium ions. A kinetic study showed that the adsorption followed the pseudo-second order kinetic adsorption model. Both Langmuir and Freundlich models described the adsorption isotherms well at pH<7.00, while the surface precipitation model showed good agreement with the experiment data when deposition occurred. The adsorption capacity, based on the unit weight and the unit surface area, was the larger for the oxidized MCFs than those for the un-oxidized ones. When the equilibrium concentration was 2 mg · L^-1 and pH=5.50, the adsorption capacities were 5.7 mg · g^-1 or 0.058 mg · m^-2. These suggested that the MCFs have potential applications in environmental protection.