Introduction: Diffusion-weighted MRI provides a non-invasive means to characterize the microstructure of biological tissues. In white matter (WM), the transverse diffusivity has been shown to be sensitive to axon size and hence allows the possibility of measuring axon size distributions quantitatively (1). However, conventional diffusion measurements use pulsed gradient spin echo methods with relatively long diffusion times, and require high b values to increase sensitivity to smaller axons, which in turn significantly decreases SNR and increases total scanning time. In the current work, an oscillating gradient spin echo (OGSE) method was used to acquire temporal diffusion spectra with relatively short diffusion times and low b values. The results show that the temporal diffusion spectrum is sensitive to the mean axon size of WM tracts of fixed rat spinal cord, ranging from very small (~1.43μm) to large (~5.26μm) axons. A new parameter R, i.e. dispersion rate of ADC vs gradient frequency, is suggested as a sensitive indicator of mean axon size. Methods: Theory: The ADC of water obtained by the cosine-modulated OGSE method can be expressed analytically inside various geometries (2), namely