Skilling was sentenced last October to more than 24 years in prison for his role in the collapse of Enron Corp., once the nation's seventh-largest company. He was convicted along with company founder Kenneth Lay on May 25, 2006, on 19 counts of fraud, conspiracy, insider trading and lying to auditors.

Skilling reported to a federal prison in Minnesota in December. Lay died before his sentencing date.

Skilling is the highest-ranking executive to be punished for the accounting tricks and shady business deals that led to the loss of thousands of jobs, more than $60 billion in Enron stock value and more than $2 billion in employee pension plans after the company imploded in 2001.

"Profound, inherent weaknesses in the government's case - not just gaps in its evidentiary proof, but doubts about its basic theories of criminality - motivated the government to resort to novel and incorrect legal theories, demand truncated and unfair trial procedures, and use coercive and abusive tactics," Skilling's lead lawyer, Daniel Petrocelli, said in the appeal filed Friday with the 5th U.S. Circuit Court of Appeals in New Orleans.