Abstract: In the theoretical framework of the relativistic mean field, the influences of π^- meson condensate on the equation of state and the particle fractions of neutron star matter including delta isobars are studied. The numerical analysis shows that the existence of π^- meson condensate has an obvious influence on the particle fractions of neutron star matter. It makes the appearance of Δ^-’s postponed and the appearance of Δ⁺’s advanced. π^- meson condensate influences weakly the equation of state of neutron star matter and it can soften the equation of state slightly. However, the tendency to make the equation of state soften is of importance.

Key words: relativistic mean field, delta isobars, equation of state, π^- meson condensate