In this study two types of PAN-based continuous carbon fibers were compared: high-modulus UMT-430 and high-strength UMT-49. The dynamics of carbon fibers oxidation at a temperature of 600°C with an exposure from 1 to 6 hours was evaluated. It was found that high-strength fibers burn out faster than high-modulus. The surface of the fibers in the initial state and after annealing was investigated. Composite samples with an Al-6Mg alloy matrix, reinforced with these types of carbon fibers, were tested for three-point bending. It was found that the strength of the composite with high-modulus fiber was more than 2 times higher than strength of the composite with high-strength fiber. The samples fracture surfaces of Cf/Al composites after mechanical tests are investigated.