This article describes the swelling behavior of polyelectrolyte brushes in aqueous solutions with different salt concentrations analyzed by neutron reflectometry. Densely-grafted polyelectrolyte brushes such as poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC), poly(2-(methacryloyloxy)ethyltrimethylammonium chloride) (PMTAC), and poly(3-dimethyl(methacryloyloxyethyl)ammonium propane sulfonate) (PDMAPS) were prepared on quartz substrate by "grafting-from" method based on a surface-initiated polymerization technique. Neutron reflectivity at swollen brushes/D2O interface afforded the neutron scattering length density profiles and the corresponding volume fraction profiles based on a parabolic function to estimate the swollen brush thickness. PMTAC brush was fairly extended from the substrate surface in D2O, while it shrunk in NaCl/D2O solution due to the screening of the repulsive interaction between polycations by hydrated salt ions. In contrast, PDMAPS brush in D2O formed shrunk structure due to strong attractive interaction between sulfobetain groups, while the swollen thickness was drastically increased by addition of salt ions to D2O. Non-dependency of swollen thickness on NaCl concentration was observed in PMPC brush because of weak electrostatic interaction between phosphorylcholine groups.