The electronic structures and total energies of BaSi2-SrSi2 and BaSi2-CaSi2 systems have been calculated using the first-principle pseudopotential method to clarify the band gap tunability of BaSi2 by alloying with Sr or Ca. From an energetic consideration of the compounds where all the Bat sites or all the Ba-II sites of the BaSi2 lattice are preferentially replaced by Sr or Ca, it is expected that the Ba-1 site will be preferentially replaced by Sr rather than the Ba-II sites. Compounds where all the Ba-II sites are replaced by Sr or all the Ba-II or all the Ba-1 sites are replaced by Ca are energetically unfavorable compared to the undissolved system of BaSi2 and SrSi2 or CaSi2. The effect of the addition of Sr or Ca into the BaSi2 lattice on the gap value is different depending on the replaced sites of Ba. The replacement of Ba-1 site by Sr will broaden the band gap of BaSi2, which is consistent with the observed results. (c) 2007 Published by Elsevier B.V.