  We present the results of numerical simulations which show the formation of
blue compact dwarf (BCD) galaxies from merging between very gas-rich dwarfs
with extended HI gas disks. We show that dwarf-dwarf merging can trigger
central starbursts and form massive compact cores dominated by young stellar
populations. We also show that the pre-existing old stellar components in
merger precursor dwarfs can become diffuse low surface brightness components
after merging. The compact cores dominated by younger stellar populations and
embedded in more diffusely distributed older ones can be morphologically
classified as BCDs. Since new stars can be formed from gas transferred from the
outer part of the extended gas disks of merger precursors, new stars can be
very metal-poor ([Fe/H] < -1). Owing to very high gaseous pressure exceeding
10^5 k_B (where k_B is the Boltzmann constant) during merging, compact star
clusters can be formed in forming BCDs. The BCDs formed from merging can still
have extended HI gas disks surrounding their blue compact cores. We discuss
whether tidal interaction of gas-rich dwarfs without merging can also form
BCDs.
