The microgels with semi-interpenetrating polymer network (semi-IPN) structure based on cross-linked poly(N-isopropylacrylamide) (PNIPAM) and linear poly(acrylic acid) (PAA) were fabricated by one-step precipitation polymerization technique. The chemical composition of the microgels was determined by Fourier transformation infrared spectroscopy and elemental analysis. It was found that the PAA content inside the microgels decreased as the pH value of reaction media increased. The semi-IPN structure of the microgels was confirmed by transmission electron microscopy using uranyl acetate staining method. The pH dependent hydrodynamic diameters obtained by dynamic light scattering revealed that these microgels can reversibly shrink and swell in the pH range from 4.0 to 6.0. The semi-IPN microgels synthesized at pH 3.0 could undergo about 120 times volume change in the pH range.