We evaluate numerically the optical characteristics of a new type of microstructured optical fiber with interstitial air holes (IH-MOF). By modification of the basic hexagonal unit cell, different optical waveguide properties can be achieved. Endlessly single mode operation is obtainable from these IH-MOF structures with smaller air-hole diameter to pitch ratio (d/ <0.25) compared with conventional triangular-latticed MOFs. The zero-dispersion-wavelength is blue-shifted with the increase of interstitial air-hole size. Leaky loss and effective mode area is significantly reduced thus the nonlinearity is enhanced by a tighter confinement.