This work deals with the use of split ring resonator (SRR) as the periodic loading over dielectric slabs placed on the sidewalls of a rectangular waveguide. This type of loading creates an unusual pass-band mode below the waveguide cut-off frequency which indeed appears to be a backward type mode. By using an external ant colony optimization (ACO) algorithm, the dimensions of the unit cell have been optimized to obtain the passband at the lowest frequency. The motivation of doing so is for miniaturization and to achieve a wide bandgap separation from the ordinary dominant modal passband. The optimization was performed based on the dispersion diagrams. The behaviour of the loaded waveguide has been studied also in transmission and the backward characteristic demonstrated. Experimental results are currently in progress.