The nanostructure and its transition of polyelectrolyte brushes in the amphiphilic diblock copolymer monolayers on the water surface were investigated by X-ray and neutron reflectivity techniques. The critical brush density (cbd), where the transition between carpet-only/carpet+brush structures occurs, was quantitatively determined and its dependencies on the hydrophilic chain length and salt concentration were examined. The cbd for polymethacrylic acid (PMA) brush decreased with increasing PMA chain length. The cbd for polystyrenesulfonate (PSS) showed no chain length dependence but increased beyond the critical salt concentration. The cbd for polyacrylic acid (PAA) showed no chain length and no salt concentration dependencies. Hence the brush formation mechanism is suggested to be different from each other for these three polyelectrolyte brushes. The salt effect on PSS brush and small ion transportation from/to the PSS brush layer were also discussed. Ref. P.Kaewsaiha, K.Matsumoto, H.Matsuoka, Langmuir, 23(1), 20-24 (2007).