A novel technique is proposed to modulate the matrix structure of poly(N-isopropylacrylamide) (PNIPAAm) hydrogel by using water and tetrahydrofuran as a mixed solvent during its polymerization and crosslinking reaction. SEM micrographs revealed that the resulting PNIPAAm hydrogels were highly porous and that the pores within the matrixes were interconnected. In addition, the hydrogels, especially those prepared in the mixed solvent with a high tetrahydrofuran content, were opaque. This indicates that the hydrogels exhibited heterogeneous structures. The porous and heterogeneous structures enabled the hydrogels to absorb large amounts of water and improved their temperature sensitivity. Interestingly, the heterogeneous hydrogels also exhibited very rapid deswelling as well as reswelling kinetics. The DSC thermograms showed that the hydrogels prepared with higher tetrahydrofuran contents had slightly greater LCSTs and enthalpies, which accounts for the increased deswelling rates. The structures and properties of the heterogeneous hydrogels were modulated by varying the tetrahydrofuran content in the mixed solvent.