The move came a day after the union at market leader Hyundai Motor Co. reached a tentative deal with management over a wage package.
GM Daewoo and its union had last week tentatively agreed a salary package that included a 3.98 percent basic pay increase.
The increase is the smallest since 2003, a year after General Motors Corp. and partners took a majority stake in some of the assets of Daewoo Motor, creating unlisted GM Daewoo.
GM Daewoo's union held a partial strike on July 14 and 19, demanding an 8.55 percent rise in base salary and better working conditions, among other requests. The strike has cost GM Daewoo an estimated 2,200 vehicles in lost output, the company said.
About 47 percent of 9,158 union members voted for the pay deal, a union official said.
"Hyundai's deal seemed to influence members, although we are still seeking the reason why members rejected the deal," the official, who declined to be named, told Reuters by telephone.
Late Wednesday, Hyundai and its union signalled agreement over a wage package that included an average 5.1 percent rise in basic salaries.
The official said GM Daewoo's union would meet again on Friday to decide future action. GM Daewoo production workers are scheduled to take summer vacation next week.
Unionized workers at Kia Motors Corp., Hyundai's affiliate and South Korea's No.2 auto maker, held a partial strike from Thursday after a similar action last week, demanding a 9.1 percent basic pay rise.