Large distortion in ISAR images of a moving target has been investigated and demonstrated under controlled experiments. The distortion is a result of small time-varying perturbed motion experienced by the target and is attributed to a modulation effect in the phase of the radar return from the target. The Doppler motion of the target, modulated by a time-varying perturbation, results in a broad smear in the phase of the radar signal. A numerical model of the distortion based on the phase modulation effect has been developed. Simulated ISAR images produced by this model are in good agreement with experimental results. The numerical and experimental studies described provide a better understanding of the distortion mechanism that affects the ISAR images of a target in motion.