Abstract: The relationships between the mechanical properties and the microstructures of typical polyacrylonitrile-based carbon fibers were studied on the basis of the elastic wrinkle model and Griffith microcrack theory. The effects of microcrystallite structure and its preferred orientation on tensile modulus, the effects of microvoids and mass density changes on the tensile strength, and the effects of internal residual compressive stress on the strain to failure, were analyzed, based on which the structural factors played a key role in determining the mechanical properties of the fibers.