The focus of the work was to clarify the influence of the cross section shape of the viscose fibers and additives on the carbonization process and the resulting carbon fiber properties. Overall, the shape of cross section of carbon fibers produced was determined by the initial fiber morphology of the precursor viscose fibers. The carbon yield was independent of the geometric structure of the viscose fibers and a certain degree of shrinkage was observed for all types of fibers due to the loss of water, hydrocarbons and other volatile products. Several additives, such as carbon black, a nitrogen containing carbohydrate and lignin, were homogenously incorporated into the precursor fiber and resulted in slight increase of the carbon yield. Carbon black particles became distinctive and visible on the surface of the carbon fibers. In particular, the thermal and chemical course of the carbonization was altered by the nitrogen containing additive, while the structure and properties of the carbonized fibers remained basically unchanged.