The United Auto Workers (UAW) union said 61% of hourly workers had voted in favour.
GM said the deal would save it $1bn (£571m) a year and cut its long-term healthcare costs by $15bn (£8.58bn).
The proposed changes still need to be approved by a US district court, the UAW union said. Last month GM announced a third-quarter loss of $1.6bn (£915m), compared with a profit of $315m (£180.2m) for the same period a year ago.
There was further bad news from GM earlier this week when it said it would have to restate its 2001 financial results by between $300m (£171.6m) and $400m (£229m).
GM said the restatement was needed because of an accounting error that caused net income figures to be exaggerated by 25%-35%.