  The Nambu--Jona-Lasinio (NJL) model is one of the most useful tools to study
non-perturbative strong interaction matter. Because it is a nonrenormalizable
model, the choosing of regularization is a subtle issue. In this paper, we
discuss one of the general things of the regularization in the NJL model, which
is whether we need to use the regularization on the thermal part by evaluating
the quark chiral condensate and thermal properties in the two-flavor NJL model.
The calculations in this work include three regularization schemes that contain
both gauge covariant and invariant schemes. We found no matter which
regularization scheme we choose, it is necessary to use the regularization on
the thermal part when calculating the chiral condensate related physics
quantities and do not use the regularization on the thermal part when
calculating the grand potential related physical quantities.
