The accident happened around 2 pm at Devil’s Elbow between Holywood and Bangor on a twisting stretch of the A2 carriageway.
The 75-year-old woman is understood to be from Groomsport in Co Down.
The road has been closed for several hours as police try to establish how the accident happened.
A man in the other the car has been taken to hospital but his injuries are not believed to be life threatening.
Motorists have been advised to avoid the area and seek an alternative route.