In this study, we conducted a thorough experimental and computational investigation to extend the pillar-splitting technique, developed initially for ceramics, to brittle fibers in reinforced composites. In pillar-splitting methodology, micropillars with circular cross-sections are tested to evaluate the load required to cause instability. The instability load is then correlated with the fracture toughness using the parameter γ which is evaluated using the cohesive zone finite element method (CZ-FEM). In reinforced composites, a unique opportunity arises due to the existence of already present micropillars (reinforcing fibers) for fracture toughness evaluation. In this work, the authors successfully applied the pillar-splitting technique to carbon fibers by investigating the effects of fiber/matrix assembly and the shape of the fiber upon the error in instability load. The minimum height required to avoid matrix effects is found to be equal to the diameter of the fiber reinforcements for a fiber–matrix system with a perfect interface. By contrast, the required protruding height is reduced to zero for a weak fiber–matrix interface. A new parameter, β, is introduced to quantify the deviation of the cross-sectional shape from circularity. An effective γ is then evaluated and used in the pillar-splitting experiments to correlate the instability load to Mode I fracture toughness. Finally, the results are validated experimentally, and the Mode I fracture toughness is evaluated for a range of Toray PAN carbon fibers. Fracture toughness is found to decrease as the modulus of the carbon fibers increases.