An experimental technique for directly measuring strain‐induced stiffening effects in individual carbon fibers has been developed. The technique involves the superposition of a small amplitude oscillating strain (&Dgr;&egr;∼10−4) on a slow linearly increasing strain (∼1.1×10−3s−1). Synchronous detection of the resultant ac component of the measured stress yields a signal proportional to the slope of the stress versus strain curve provided the modulation amplitude is small compared to the ultimate strain at failure (∼0.005). The experimental arrangement has been designed for use in conjunction with an Instron tensile test apparatus. A novel design for a fiber gripping apparatus which permits direct measurement of the strain is also described. The technique has been applied to carbon fibers produced from two different precursor materials: polyacrylonitrile and mesophase pitch. Fibers from both precursors exhibited strain‐induced stiffening. The modulus of the pitch‐based fibers increased in proportion to the square root of the strain, whereas that of the PAN‐based fibers showed an almost linear increase.