Modified loosestrife powder (MLP), as a new adsorbent, was prepared from dry loosestrife powder by oxidization treatment using peracetic acid with tetrahydrate manganese acetate as catalyst. IR spectra analysis and Boehm titration method were employed to explore MLP’ surface acidic functional groups. Batch adsorption experiments were carried out for the removal of Methylene Blue (MB) from aqueous solution by MLP. Equilibrium experimental data were better represented by Freundlich isotherm using linear methods. Thermodynamic parameters such as G, H and S were also calculated. The results showed that the adsorption of MB onto MLP was efficient and could serve as a promising low-cost adsorbent.