A series of poly(methacrylic acid-co-(2-hydroxyethyl methacrylate)) nanogels with the different mole percentages of monomers have been prepared by the inverse microemulsion polymerization.The actual mole percentages of monomers and pKa value of nanogels were determined.The morphology of nanogels have been characterized by transmission electron microscopy(TEM).The pH responsive property of nanogel was researched.pKa of the nanogels with 20% mole percentage of MAA is 5.83,the number-average diameters of the nanogels are 48 nm and 90 nm as the pH=3 and pH=7 respectively.When pH approaches pKa of the nanogels,the turbidity of nanogel dispersion decreases significantly with the swelling rate of nanogels rising obviously and the apparent viscosity of nanogel dispersion increasing sharply,which indicate that the nanogels have an excellent pH responsibility.