Cain's proposal would replace the current tax code with a 9 percent tax on all personal income, corporate income and sales.
The congresswoman said she agreed with lower tax rates, but doesn't believe that an added sales tax would guarantee lower overall federal tax revenues.
Other conservatives have also rejected Cain's proposal, saying it would create way for future generations to raise the tax burden on Americans.
"The last thing you do is create a brand new pipeline of a revenue stream for a, the United States Congress … once you create a new tax it will never go away," said Bachmann.
She told Fox News that she doesn't believe Congress would keep the tax rate at a flat 9 percent. Bachmann said the plan would have the potential to go from 9-9-9 to "15-15-15 or 30-30-30."
"They [Congress] have a revenue appetite. It is never enough money," she added.
Bachmann said she was also concerned a federal sales tax would be too large a burden in many places when added to already existing state and local taxes.