Terahertz radiation (a.k.a. T‐ray) has emerged as a powerful inspection technique in recent years. It extends into the lower region of far‐infrared in the electromagnetic spectrum, and has been proven very effective in many NDE applications. T‐ray is particularly useful in some areas where accessibility is difficult or even not possible for other conventional NDE methods. Here we report a modeling effort of T‐ray signal in one such application: the detection of “hidden delamination”, a special situation that a delamination was shadowed by another. This special case was experimentally simulated by two parallel saw‐cuts in glass composites. In this paper, we also explore the feasibility of combing T‐ray and Fourier transform infrared spectroscopy using a unified least‐squares scheme for material characterization.