Ed Miliband, the former energy secretary, got 50.65 percent of the vote on the fourth and final round, The Guardian reported. David Miliband, the former foreign secretary, got 49.35 percent.
Three other candidates were eliminated in the first rounds of voting.
Ed Miliband, 40, had the support of the left wing of the party, including union leaders. His older brother David, 45, who was long seen as former Prime Minister Tony Blair's heir, had more votes from members of the British and European parliaments.
During his leadership campaign, Miliband said New Labor is "dead." He denounced what he described as a turn to "brutish, U.S.-style capitalism."
Miliband is expected to ask his brother to join his shadow cabinet.
"We've never seen anything quite like what we've had, with two brothers neck and neck," said David Blunkett, the former home secretary. "These are brothers. They're blood brothers. They can't afford to fall out in the way we had with Tony and Gordon (Brown), and neither can we."