We present an analytical investigation for a baseline-free imaging of a defect in plate-like structures using the time-reversal of Lamb waves.We first consider the flexural wave (A mode) propagation in a plate containing a defect, and reception and time reversal process of the output signal at the receiver.The received output signal is then composed of two parts: a directly propagated wave and a scattered wave from the defect.The time reversal of these waves recovers the original input signal, and produces two additional sidebands that contain the time-of- flight information on the defect location.One of the side-band signals is then extracted as a pure defect signal.A defect localization image is then constructed from a beamforming technique based on the time-frequency analysis of the side band signal for each transducer pair in a network of sensors.The simulation results show that the proposed scheme enables the accurate, baseline-free imaging of a defect.