As a biofilm carrier for wastewater treatment, PAN-based carbon fiber (CF) was usually modified with different ways. The double modification treatment composed of surface oxidation and maleic anhydride (MA) graft was used in current research and it’s regarded to be a favorable surface treatment for CF carrier. The consequent CF surface morphology and natures were then characterized by laser confocal microscopy (LCM), X-ray photoelectron spectroscopy (XPS) and the degree of moisture. In addition, biocompatibility of CF was discussed through immobilization behavior of microorganisms. The results indicated that the immobilization ability of microorganisms will be improved if the CF surface is modified to better hydrophilicity by producing oxygen-based functionalities. Furthermore, the configurations and the adhesion forms of cells were affected by the properties of CF surface.