The westbound lanes of Highway 401 at Highway 6 South were closed most of the day following an earlier crash involving a car and a tractor-trailer.
Ontario Provincial Police said the initial accident happened around 5:15 a.m. on Wednesday.
When officers arrived on scene the truck was fully engulfed in flames and there was a fuel spill.
OPP Sgt. Kerry Schmidt said one person has been taken to hospital with life-threatening injuries.
No word yet on if charges will be laid.
The westbound lanes had been closed for most of the day due to repaving.