The mission to attach a new segment, brought to the orbital station by the space shuttle, lasted six and a half hours, the Huston Space Center said.
A spokesman for the center said U.S. astronaut Robert Curbeam and Christer Fuglesang, the first Swede in space, did not run into difficulties during the spacewalk.
The shuttle delivered a new cube-shaped 4,000-lb section to be attached to the ISS, and a large amount of cargo, including solar batteries.
Russian Space Agency spokesman Igor Panarin said earlier, "We hope all technical work during the three planned spacewalks will be completed to make the station's configuration more stable, and improve its capacity from the viewpoint of providing life-support and carrying out research experiments."
The two upcoming spacewalks are aimed at improving the station's power generation system by activating the Solar Alpha Rotary Joint to allow solar arrays to track the sun.
Six of the seven astronauts are set to leave the space station in a week, while the seventh, Sunita Williams, will stay for six months, together with station commander Michael Lopez-Alegria and Russian cosmonaut Mikhail Tyurin, replacing the European Space Agency's German astronaut, Thomas Reiter.
