Because numerous drugs are administered through an oral route and mainly absorbed at the intestine for its ultimate desired effects, the prediction of drug permeability across an intestinal epithelial cell membrane has been a crucial issue in drug discovery. Thus, various in vitro permeability assays have been developed such as the Caco-2 assay, the parallel artificial membrane permeability assay (PAMPA), the phospholipid vesicle-based permeation assays (PVPA) and Permeapad®. However, due to the laborious and quite expensive process for cell culture in the Caco-2 assay and the uncertain microscopic membrane structures of the other assays, a simpler yet more accurate and versatile technique is still required. Here, we developed a new technique to measure the permeability of drug molecules across a planar freestanding lipid bilayer with a large area and a well-defined structure. The lipid bilayer was constructed within a conventional UV spectrometer cell, and the transport of drug molecules across the bilayer was recorded by UV absorbance over time. Permeability was then computed from the time-dependent diffusion equation. We measured the permeability for several exemplary drugs and compared these values with previously reported ones. We found that our assay has a much higher permeability compared to the other assays, and this higher permeability is related to the thickness of the lipid bilayer. Also we were able to measure the dynamic permeability upon the addition of a membrane-disrupting surfactant demonstrating that our technique has the capability to detect real-time changes in permeability across the lipid bilayer.