We study the effects of temperature on the magnetic field of neutron star under relativistic mean field theory. The field is produced under the assumption that the photon has gained its mass in dense nuclear matters. We have computed the field at several temperatures and get a right order of magnitude. We also conclude that a cooler neutron star has a stronger magnetic field. We work in the framework of the nonlin- We study the effects of temperature on the magnetic field of neutron star under relativistic mean field theory. The field is produced under the assumption that the photon has gained its mass in dense nuclear matters. We have computed the field at several temperatures and get a right order of magnitude. We also conclude that a cooler neutron star has a stronger magnetic field. We work in the framework of the nonlin-