Abstract: Nano-sized magnesium hydroxide［Mg（OH）₂］ was modified by sodium dodecyl sulfate（SDS）/monoalcoholether phosphate（MAP） combinational surfactant as modifier.When the mass ratio of SDS and MAP was 1∶1，the influence of the dosage of combinational surfactant on the performance of nano-sized Mg（OH）₂ was investigated by sample analysis with XRD，TEM，FT-IR，BET，and determination of liquid suspension product in liquid paraffin etc.，and a preliminary discussion about the mechanism of modification was also made. Results showed the hydrophobic properties of the sample′s surface increased firstly and then decreased in trend with the increasing of the dosage of SDS/MAP and reached maximum when the dosage was 0.2% of the sample′s weight；the particle size followed the decrease-increase-decrease variation law and the inflexion points appeared when the dosages were 0.2% and 0.5% respectively；the variation law of surface area was the same as the particle size.It was showed that SDS/MAP could be absorbed onto the surface of Mg（OH）₂ in the form of mono-layer or bi-layer in water depending on the dosage of SDS/MAP.

Key words: sodium dodecyl sulfate, monoalcoholether phosphate, nano-sized magnesium hydroxide, function mechanism