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Continuous carbon fiber epoxy-matrix composites are important for lightweight structures. Fastening is a common method of joining composite components. In the case of composite laminates, fastening involves compression in the direction perpendicular to the laminae. The effect of fastening on the microstructure of composite laminates is of current aircraft safety concern, due to the 2001 Airbus accident in New York. The accident involved degradation of the fastening between the tail and the body of the aircraft. Considerable prior attention has been given to studying the effect of tension in the fiber direction of a composite on the microstructure of the composite. This is because the fiber direction is the strong direction of the composite and the composite is stronger under tension than compression in this direction. In contrast, this paper is focused on the effect of compression in the throughthickness direction, due to its relevance to fastening. Due to the electrical conductivity of carbon fiber, electrical resistance measurement is effective for monitoring the microstructural changes in a carbon fiber composite. This method is also advantageous in its nondestructive nature, fast response, equipment simplicity and applicability to large structural components. This paper uses DC electrical resistance to monitor the effect of repeated compression in the through-thickness direction at various stress amplitudes (equivalent to repeated fastening and unfastening at various loads) on the microstructure of the interlaminar interface (i.e., the interface between adjacent laminae) and on that within a lamina. The effect on the microstructure of the interlaminar interface was studied by measuring the contact electrical resistivity of the interface. The effect on the microstructure of a lamina was studied by measuring the volume resistance of a lamina in both the longitudinal and transverse directions.