In this work, a facile one-pot strategy was proposed for the synthesis of C18-functionalized core-shell magnetic mesoporous silica composite (Fe3O4/mSiO2-C18). The Fe3O4/mSiO2-C18 composite, with an average size of 80 nm and a functionalized mesoporous silica shell of about 30 nm in thickness, has excellent adsorption ability toward methylene blue dye (MB) due to the large surface area (303 m(2) g(-1)) and the abundant hydrophobic C18 groups. The adsorption equilibrium was achieved within 20 min and the adsorption behavior of MB on Fe3O4/mSiO2-C18 composite fitted the pseudo-second-order kinetic model well (k2=1.29×10(-2) g mg(-1) min(-1), q(e)=144.72 mg g(-1), h(o)=270.27 mg g(-1) min(-1) under 25 °C and an initial MB concentration of 10 mg L(-1)). Langmuir and Freundlich isothermal adsorption models can both be used to describe the adsorption process and the maximum Langmuir adsorption capacity of MB on Fe3O4/mSiO2-C18 at 25 °C and pH 7.5 is 363.64 mg g(-1). Thermodynamic parameters show that the adsorption reaction is exothermic and spontaneous (ΔH(0)=-63.49 kJ mol(-1), ΔG(0)=-7.80 kJ mol(-1)). Ionic strength and pH affected the adsorption slightly. In addition, the MB adsorbed sorbent can be readily separated from water solution by an external magnet because of the high magnetic saturation value (22.62 emu g(-1)). After being regenerated by treatment with acidic methanol, the sorbent could be reused for at least 5 cycles with a little decrease in adsorption capacity.