  In the mean field approximation of the relativistic $\sigma$-$\omega$-$\rho$
model, the magnetic fields are incorporated, and its influence on the
properties of n-p-e neutron star matter are studied. When the strength of the
magnetic field is weaker than $\sim 10^{18}$G, the particle fractions and
chemical potentials, matter energy density and pressure hardly change with the
magnetic field; when the strength of the magnetic field is stronger than $\sim
10^{20}$G, the above quantities change with the magnetic field evidently.
Furthermore, the pressure is studied in both thermodynamics and hydrodynamics.
The difference between these two ways exits in the high density region, that
is, the thermal self-consistency may not be satisfied in this region if the
magnetic field is considered.
