  We present a theoretical model for supernova (SN) 2008D associated with the
luminous X-ray transient 080109. The bolometric light curve and optical spectra
of the SN are modelled based on the progenitor models and the explosion models
obtained from hydrodynamic/nucleosynthetic calculations. We find that SN 2008D
is a more energetic explosion than normal core-collapse supernovae, with an
ejecta mass of Mej = 5.3 +- 1.0 Msun and a kinetic energy of E = 6.0 +- 2.5 x
10^{51} erg. The progenitor star of the SN has a 6-8 Msun He core with
essentially no H envelope (< 5 x 10^{-4} Msun) prior to the explosion. The
main-sequence mass of the progenitor is estimated to be Mms =20-25 Msun, with
additional systematic uncertainties due to convection, mass loss, rotation, and
binary effects. These properties are intermediate between those of normal SNe
and hypernovae associated with gamma-ray bursts. The mass of the central
remnant is estimated as 1.6 - 1.8 Msun, which is near the boundary between
neutron star and black hole formation.
