The effects of fibre/matrix bonding,fabric density,fibre volume fraction and bundle size on microstructure, mechanical properties and failure mechanisms in carbon fibre reinforced composites (plastic and carbon matrix) have been investigated.The microstructure of unloaded and cracked samples was studied by optical microscopy and scanning electron microscopy (SEM),respectively whereas the mechanical behaviour was examined by 3- point bending experiments.Exclusively one type of experimental resole type phenolic resin was applied.A strong fibre/matrix bonding,which is needed for high strength of carbon fibre reinforced plastic (CFRP) materials leads to severe composite damages during the pyrolysis resulting in low strength,brittle failure and a very low utilisation of the fibres strain to failure in C/C composites.Inherent fabric parameters such as an increasing fabric density or bundle size or a reduced fibre volume fraction introduce inhomogenities to the CFRP’s microstructure.Results are lower strength and stiffness whereas the strain to failure increases or remains unchanged.Toughness is almost not affected.In C/C composites inhomogenities due to a reduced bundle size reduce strain to failure,strength,stiffness and toughness.Vice versa a declining fibre volume fraction leads to exactly the opposite behaviour.Increasing the fabric density (weight per unit area) causes similar effects as in CFRPs. The effects of fiber / matrix bonding, fabric density, fiber volume fraction and bundle size on microstructure, mechanical properties and failure mechanisms in carbon fiber reinforced composites (plastic and carbon matrix) have been investigated. The microstructure of unloaded and cracked samples was studied by optical microscopy and scanning electron microscopy (SEM), respectively, the experimental behavior was examined by 3-point bending experiments. One type of experimental resole type phenolic resin was applied. A strong fiber / matrix bonding, which is needed for high strength of carbon fiber reinforced plastic (CFRP) materials leads to severe composite damages during the pyrolysis resulting in low strength, brittle failure and a very low utilization of the fibers strain to failure in C / C composites.Inherent fabric parameters such as an increasing fabric density or bundle size or a reduced fiber volume fraction to inhomogenities to the CFRP’s microstructure. Results a re lower strength and stiffness and the stiffness to the strain to failure increases or remains unchanged. Toughness is almost not affected. Cc C composites inhomogenities due to a reduced bundle size reduce strain to failure, strength, stiffness and toughness. Verb versa a declining fiber volume fraction leads to exactly the opposite behavior.Increasing the fabric density (weight per unit area) causes similar effects as in CFRPs.