To address a long-standing issue of building molecular electrical circuits heavily relying on anchoring chemistry, we propose an alternative method to immobilize anchor-free molecules in a molecular cavity between metal electrodes. In such a scheme, well-defined conductance distribution of anchor-free molecules was obtained by means of a scanning tunneling microscopy break-junction technique. Density functional theory calculations suggest that effective electronic coupling at the molecule-electrode interface can be achieved through well-defined van der Waals (vdW) interactions when the molecule is confined in a cavity with a defined geometry. This work offers a new paradigm to achieve reliable conductance measurements of molecules via vdW interaction without metal-binding groups.