Random copolymer hydrogel actuators, composed of poly(acrylic acid) and poly(vinyl sulfonic acid, sodium salt), were prepared. The swelling ratios at various temperatures and pHs, the deswelling water ratio and contraction/expansion behavior under an electric field for the hydrogel actuators were measured. The hydrogels exhibited very high swelling ratios, in the range of 8200 ~ 18000%, at 37 °C, and showed temperature/pH dependent swelling behavior. The deswelling water ratio of the CO1 hydrogel sample showed about an 80% weight reduction under a 5 V applied voltage. When the hydrogel actuator in various pH buffer solutions is subjected to an electric field, the hydrogel actuator was contracted. When the electric stimulus was removed, the hydrogel actuator was expanded on its original size. The hydrogel actuator also showed stepwise contraction/expansion behavior depending on the electric stimulus.