The optimization of a nuclear converter core in respect of thermal power, power peaking and fuel conversion ratio is studied from the aspect of neutron flux distribution. It is shown that flattening the neutron flux distribution maximizes the thermal power output. A mathematical approach to the problem of flattening the neutron flux distribution is presented for a regionized reactor. Minimization of power peaking at the border of the region of flattened flux is considered with one- and two-group approximations. The analyses on one dimensional model has led to the result that the optimal reactor should be three-regional.