A novel hybrid waveguide structure of rectangular silicon insert two identical nuclei in a rectangular metal trench is presented. Based on the finite element method, variations of effective mode area and propagation distance with distance between the two nuclear medium, nuclear medium height and width, the distances between dielectric core and metal are systematic studied at a wavelength of 1550 nm. Calculated results show that the enhanced effect of field in gaps of the dielectric core or between dielectric core and metal can get a low loss hybrid mode of ultra-small modal area. When the distances between dielectric core and metal is wide, the gap in dielectric core is 5 nm, effective mode area of the structure is drastically reduced to about one-seventh respect to the nuclear medium with no gap and the propagation distance increasing a slight remains at 50 wavelengths around. And the smaller the mode area is, the longer the propagation distance is. The higher the gap dielectric core is, the smaller the propagation distance is, and the mode area is almost invariably. When the height of dielectric core is higher, the propagation distance is longer, but the effective mode area changes minor. When the distance between dielectric core and metal is narrow, the gap of nuclear medium and nuclear medium width are much narrower, the effective mode area are smaller.