The increase was driven by a net increase in the number of firms who expect better business conditions six months from now. Companies also were more optimistic about revenues and profits.
NFIB economist William Dunkelberg noted that the index remains well below pre-recession levels, and is also below the highest levels reached in 2013.
(Read more: Six years post-recession, a tale of Wall St. and Main St.)
Still, the index has trended upward over the last year, and December showed generally positive signals.
"Reports of capital spending did rise significantly in December... and plans to make expenditures did rise a bit," Dunkelberg said.