  For some neutron stars (NSs) in the binary systems, the masses have been
accurately measured. While for the isolated neutron stars (INSs), no mass
measurement has been reported yet. The situation will change soon thanks to the
successful performance of the Neutron Star Interior Composition Explorer
(NICER), with which the radius and mass of the isolated PSR J0030+0451 can be
simultaneously measured. For most INSs, no mass measurements are possible for
NICER because of observational limitations. Benefiting from recent significant
progress made on constraining the equation of state of NSs, in this work we
propose a way to estimate the masses of the INSs with the measured
gravitational redshifts. We apply our method to RX J1856.5-3754, RX
J0720.4-3125, and RBS 1223, three members of "The Magnificent Seven" (M7), and
estimate their masses to be $1.24_{-0.29}^{+0.29}M_{\odot}$,
$1.23_{-0.05}^{+0.10}M_{\odot}$, and $1.08_{-0.11}^{+0.20}M_{\odot}$,
respectively. These masses are consistent with that of binary NS systems,
suggesting no evidence for experiencing significant accretion of these isolated
objects.
