"We're looking at issuing a 50-year bond, what we could call an ultra-long bond. We think there is some demand for it. It is something we'll very seriously consider for next year," Xinhua news agency quoted Mnuchin as saying in a CNBC interview on Thursday.
"I personally think it would be a good thing to expand the US borrowing capabilities," Mnuchin said.
"It's obviously quite attractive for us to extend and de-risk the U.S. Treasury borrowing."
Munchin said later in the day that the US Treasury could consider the possibility of 100-year bonds if 50-year bonds were issued and successful.
His remarks come a day after President Donald Trump tweeted on Wednesday that the US should start to refinance its debt, which has exceeded a record $22 trillion.
Mnuchin and his Department have been studying the possibility of issuing ultra-long bonds since he first took office in 2017.
Currently, the Treasury only issues 10-year and 30-year long-term bonds.
But the Treasury Borrowing Advisory Committee of the Securities Industry and Financial Markets Association (SIFMA), a government-appointed panel of external experts, concluded in 2017 that "there is little evidence of strong or sustainable demand" for ultra-long bonds in the US markets.
The advisory committee also noted that foreign demand for ultra-long bonds was likely to be low, as foreign holdings of US Treasuries are predominantly shorter maturity.
( With inputs from IANS )