Due to the advances in optical trapping technology, it has become possible to trap a molecule in nanometer-size scale beyond a few micrometer scale. In particular, as plasmonic devices have begun to be used in optical trapping technology, it has become possible to tightly trap light into smaller spaces, making trapping forces larger. In this paper, we show that trapping of a nanoscale molecule can be achieved by using 3D plasmonic nano cavity through quantum dot trapping experiments with a very small size of diameter 3.3 nm. We also shows that it is possible to trap DNA with a diameter of 2 nm and a length of 7 nm which is known to have a low refractive index. Due to the structure of the optical resonator fabricated by our laboratory, dynamic movement of molecules can be understood by observing a nonlinear phenomenon as well as the one-dimensional transmission signal.