It happened after 9:00 a.m. Wednesday at the Claymont Steel Mill, located in the 4200 block of Philadelphia Pike.
Officials tell Action News the 44-year-old worker was performing maintenance on a crane when he became pinned between the crane and a catwalk approximately 50 feet above the ground.
It took rescue crews about an hour to free the man.
He was lowered to the ground and transferred to a Delaware State Police helicopter for transport to Christiana Hospital.
Officials say the man suffered chest injuries but was conscious and alert during transport.
He was admitted to the hospital in serious condition but stable condition.
OSHA is investigating the incident.