Traffic ground to a halt near Wolverhampton after the blaze broke out at around 9am.
Smoke was pouring into the sky over the busy motorway as flames leaped out of the roof of what is believed to have been a flatbed lorry.
Nobody is believed to have been injured.
Both lanes were closed between Junction 2 for Wolverhampton and Junction 3 for Cosford, with drivers being diverted passed the incident for around an hour.
Motorists were diverted up to Gailey and along the A5 and A41 until the motorway was back fully open at around 10.45am.
Crews from West Midlands Fire Service tackled the flames.