Both police and ambulances are on the scene for the incident, which took place between Bodmin and Fraddon.
Speaking to Cornwall Live a witness commented: Massive fire on the fly over just past Roche services heading for Indian Queens. Huge tailbacks both ways.
“Huge police presence and three ambulance services.”
Photographs from the scene show a small truck burning on the flyover, with police having blocked the A30 below.
Devon and Cornwall Police told Cornwall Live they were first notified about the incident at 6.32pm.
A spokesman said: “We are currently dealing with a vehicle fire on a road over the A30 near Trevose.
“We are on the scene with the fire service and awaiting more updates from the scene.”
Security is tight across Cornwall ahead of the G7 meeting which begins in the county on Friday.
A number of world leaders, including Joe Biden and Justin Trudeau, have travelled down the A30 in motorcades over the past 24 hours.
More to follow…