It happened at a home in Lower Nazareth Township, Northampton County, just after 6 p.m. Monday night.
Police say the 6-year-old boy's father accidentally backed over him with a riding lawn mower, trapping the child's leg in the blades.
The dad was able to lift the machine up and apply a tourniquet to control the bleeding, but the boy still suffered life-threatening injuries.
He's now being cared for at Lehigh Valley Hospital.