It happened around 7:30 p.m. Wednesday in the town of Maine.
The sheriff's department says the man and his teenage son were working on the lawn mower, when something ignited the fuel. Both suffered significant burns.
The father was flown to a Madison hospital. The son was taken by ambulance to Theda Clark Medical Center in Neenah. Their conditions are not being released.
Investigators are still trying to figure out what set off the explosion.