We propose new design parameters for few mode index-guiding holey-fiber (IGHF) that can provide ultra-flattened dispersion properties as well as adiabatic mode transformation capability. A novel silica index guiding holey fiber (IGHF) design is proposed utilizing a new hollow ring structure that is composed of germanosilicate high index ring and hollow air hole imbedded in a triangular lattice structure. The proposed IGHF showed unique modal properties such as nearly zero flattened dispersion over a wide spectral range with low dispersion slope by flexible defect parameter control. It is predicted that ultra-flattened dispersion of 0±0.5ps/(km.nm) from wavelength 1360nm to 1740nm could be achieved with a slope less than 1.10-3ps/km.nm 2, along with fine tuning ability of dispersion value. In contrast to prior IGHF, the proposed fibers can be achieved adiabatic mode transformation from annulus mode to a mode generated from solid multi-core fiber due to germanosilicate rings that is highly compatible to LP01 mode in conventional step index fiber. This adiabatic mode conversion of optimized IGHF for ultra-flattened dispersion contributed to low splicing loss, 0.01 dB at 1550nm to dispersion compensation fiber.