Nowadays, cellulose fibers are widely used for paper, textile, and hygiene products. While paper is produced using native cellulose fibers extracted from wood, cellulose fibers for textiles and hygiene products are manufactured from regenerated cellulose fibers. Native cellulose fibers are called cellulose type I fibers, whereas regenerated cellulose fibers are called cellulose type II fibers. The raw material for both fiber types is wood. Native cellulose fibers are wood cells which are composed of cellulose, hemicelluloses and lignin and are extracted from the wood matrix by the pulping process. Regenerated cellulose fibers are produced by dissolving the cellulose from pulp and spinning it into fibers. The native cellulose fibers investigated in this work were spruce and pine kraft pulp fibers, provided by an industrial supplier. The pulp fibers were treated at different temperatures, which led to a decrease in mechanical properties of a formed sheet. The fiber surfaces were characterized by atomic force microscopy (AFM) in wet and dry state. Dried pulp fibers show a wrinkled surface. Pulp fibers in wet state - measured in an aqueous environment - reveal a smoother surface with only a few wrinkles. Also single microfibrils with a diameter of about 120 nm and lignin precipitates could be observed in AFM topography images. On spruce pulp fibers, the investigated surface layer was identified as the secondary wall number one, whereas on pine pulp fibers the primary wall was found to be the exposed layer. Additionally, pine pulp fiber surfaces showed a higher coverage of lignin precipitates than spruce pulp fiber surfaces. The investigated regenerated cellulose fibers were viscose type fibers directly taken from the production line. Classical viscose fibers with a cloud-shaped cross-section and fibers with a collapsed, hollow cross-section were characterized. One of the hollow viscose fiber samples was modified by carboxymethyl cellulose (CMC). This modification increased the negative surface charge. On the hollow fibers, surface wrinkles in the range of 500 nm to 1000 nm were observed. The classical viscose fibers showed surface wrinkles with a size of about 3000 nm. Also, the fibrillar fine structure was visualized using AFM phase imaging in tapping mode where a fibril width between 30 nm and 40 nm was obtained. The amount of CMC - determined by AFM - on the scanned region of the modified sample was three times the added amount.