We present a novel design approach for realizing holey fibers (HFs) with flat dispersion characteristics and large mode area based on the existence of an artificially defected air-hole ring in the cladding, and on the inclusion of additional defected air holes in the core of the fiber. This unique type of HF can be used for achieving remarkable flat dispersion characteristics as well as a large mode area, which are particularly useful for high-speed data transmission. The validation of the proposed design is done by adopting an efficient full-vectorial finite element method for optical characterization of HFs. The proposed fiber can be employed in reconfigurable optical transmission systems for performing wavelength division multiplexing operation. Typical characteristics of the proposed HF are a flattened dispersion of $6.3\pm 0.5\mathrm{ps}∕\mathrm{km}∕\mathrm{nm}$ from $1.45\text{to}1.65\mu \mathrm{m}$ and an effective mode area as large as $100\mu {\mathrm{m}}^2$ in the same frequency range.