This proposesa metal nanostructure composed of a metal nanodisc and a metal nanosphereon top of it, which canbe applied for surface enhanced Raman scattering. Due to the excitation of the breathing mode surface plasmon resonance of the nanodisc, this nanostructure can form a gap mode with efficient longitudinal electric field enhancement under the illumination of a radially polarized vector beam. The simulation based on finite element method is carried out to investigate this gap mode and an electrical filed enhancement of 100 times relative to the valid transverse electrical field is demonstrated. In order to present more clearly the spectrum characteristic and the surface electric field distribution of this new nanostructure, the other structures including a single metal nanodisc, a single metal nanosphere, metal film and a metal nanosphere-on the metal film-are also studied under the same simulation configuration. Since the metal nanosphere can be regarded as the tip of a metal probe, the gap mode proposed here is expected to find application in tip enhanced Raman scattering.