The thermal conductivity κ(T) has been measured for sintered-La 0.50 Sr 0.50 MnO 3 (S-LSMO) and Nd 0.50 Sr 0.50 MnO 3 fabricated by the floating zone method (FZ-NSMO) in magnetic fields up to 5T. Both crystals commonly show a first order transition from the ferromagnetic (FM) to the antiferromagnetic (AF) phase with decreasing temperature and κ(T) shows seemingly similar reductions just below the Néel temperature T N in zero magnetic field. The field dependence of κ(T) is, in contrast, quite different. κ(T) of FZ-NSMO increases over the entire temperature range below T N in applied fields because of the electronic-component (κ e ) enhancement. For S-LSMO, where the heat conduction is mainly due to phonons, κ(T) hardly depends on the magnetic field except for the change corresponding to the T N shift. Analyses suggest that the phonon scattering by conduction electrons is very strong in the metallic FM phase of FZ-NSMO.