  Motivated by the discovery of extremely bright supernovae SNe1999as and
2006gy, we have investigated how much 56Ni mass can be synthesized in
core-collapse massive supernovae (SNe). We calculate the evolution of several
very massive stars with initial masses M <~ 100 Msun from the main-sequence to
the beginning of the Fe-core collapse and simulate their explosions and
nucleosynthesis. In order to avoid complications associated with strong
mass-loss, we only consider metal-poor stars with initial metallicity Z =
Zsun/200. However, our results are applicable to higher metallicity models with
similar C+O core masses. The C+O core mass for the 100Msun model is M_CO =
42.6Msun and this is the heaviest model in the literature for which Fe-core
collapse SN is explored. The synthesized 56Ni mass increases with the
increasing explosion energy and progenitor mass. For the explosion energy of
E_51 = E/10^{51} erg =30, for example, the 56Ni masses of M(56Ni) = 2.2, 2.3,
5.0, and 6.6 Msun can be produced for the progenitors with initial masses of
30, 50, 80 and 100 Msun (or C+O core masses M_CO = 11.4, 19.3, 34.0 and 42.6
Msun), respectively. We find that producing M(56Ni) ~ 4Msun as seen in SN1999as
is possible for M_CO >~ 34 Msun and E_{51} >~ 20. Producing M(56Ni) ~ 13Msun as
suggested for SN2006gy requires a too large explosion energy for M_CO <~
43Msun, but it may be possible with a reasonable explosion energy if M_CO >~
60Msun.
