Cellulose is one of the most important renewable raw materials in recent time and also the most frequently occurring organic compound in the world. This raw material can be used to produce either paper or textiles. In the first case the cellulose is called native cellulose or cellulose type I. In the second case the cellulose is known as regenerated cellulose or cellulose type II. Atomic force microscopy (AFM) was applied to characterize these two types of cellulose fibers. The first part of the diploma thesis deals with the investigation of paper fibers. Fibers from the Mondi Packaging Frantschach GmbH, St. Gertraud, Austria, were used to analyze the morphological behavior of the surface. The surface arrangement is beside the chemical composition the most important influence on the fiber-fiber bond. With AFM phase imaging it was possible to show the arrangement of micro fibrils with diameters of about 40 nm to main bundles with diameters of about 300 nm. It was also possible to reveal the influence of the drying and ozone treatments on the fibril arrangement. The second part deals with the investigation of the inner structure of textile fibers, which has - beside the chemical composition - the greatest influence on the macroscopic properties. The investigation focused to analyze the distribution of pores, their size and shape. Cross-sectional scans on Modal, Viscose and TENCEL(R) fibers from Lenzing AG, Lenzing, Austria, were done. To achieve a three-dimensional view of the fiber structure additional measurements on longitudinal cross-sections were performed. By means of AFM phase imaging, it was possible to reveal the different pore structure within the individual fibers on the nanometer scale and to determine quantitatively the pore size.