The simplified method for the synthesis of thermosensitive porous hydrogels by a radical polymerization was presented and their swelling properties were examined experimentally. N,N-Diethylacrylamide (DEAAm), or N-isopropylacrylamide (NIPAm) as primary monomers and N,N＇-methylenebiscarylamide (BIS) as a crosslinker were used. They were polymerized in water at various temperatures above the lower critical solution temperature (LCST) of poly-DEAAm (ca. 32 degrees C) or poly-NIPAm (ca. 31 degrees C) by using N,N,N＇,N＇-,tetramethylethylenediamine (TEMED) and ammonium peroxsodisulfate (APS) as the polymerization accelerator and initiator, respectively. From the observation by a scanning electron microscope, it was found that these gels consisted of aggregated microgel particles, namely, a porous structure. The gels swelled below their LCSTs, and the swelling degree increased with lowering temperature. Furthermore, the gels swelled or shrank very fast in response to the change in temperature, and the shrinking rate was larger than the swelling rate. Such swelling properties and mechanical properties depended on the porous structure of the gels such as the size of the microgels and the pore volume, which largely changed with the synthesis temperature and the component of the primary monomer.