"The vote of confidence was obtained by 83 votes out of 94 lawmakers present," MP Mohammed Laamari told AFP.
Miitig's cabinet is due to replace that of Abdullah al-Thani, who resigned last month amid mounting lawlessness, and as a rogue general gains support for an assault against Islamist militias in the east of the country.
The GNC elected Islamist-backed businessman Miitig, 42, prime minister in a chaotic vote in early May. The session came just days after gunmen stormed the GNC to interrupt an earlier ballot.
Since Kadhafi's fall, successive governments in the oil-rich North African nation have struggled to impose order as heavily armed former rebel brigades have carved out their own fiefdoms and refused to join the security forces.
In mid-May, former general Khalifa Haftar launched an offensive against Islamist militias in the eastern city of Benghazi, cradle of the 2011 uprising and a hotbed of attacks against troops and police.
Army units and officers have joined Haftar's forces, and on Friday thousands of demonstrators rallied in support of his assault.