The wavelength of terahertz radiation is typically on the order of millimeters and submillimeters, and not negligible compared to the sizes of the optical components and the sample structure. Diffraction and scattering effects will be prominent in this situation, yielding spatiotemporal reshaping of terahertz pulse during its propagation. One representative example involves the propagation of a focused terahertz beam through a dielectric edge, which is frequently encountered in terahertz pulsed imaging. In this paper, we present a theoretical and experimental investigation on this edge diffraction problem. Benefiting from the presence of edge diffraction, high contrast image is likely to be obtained, which enables a weak-absorption (or transparent) object to be clearly resolved and reveals potential applications of terahertz imaging in nondestructive detection and biomedical diagnosis. © 2009 SPIE.