The party, led by Aung San Suu Kyi, was banned or had its activities restricted under Burma's military leaders.
But it won parliamentary seats in by-elections in 2012 that came amid reforms by the new civilian government.
Aung San Suu Kyi has spoken of the need to reinvigorate the party since it won its battle for state recognition.
She wrote in the party's newspaper earlier this week that it was "necessary to strengthen the party with new blood and it is necessary to make correct choices".
The NLD, which has more than one million members across Burma, was formed in 1988 and won parliamentary elections in 1990.
But the military never allowed it to take power and imprisoned Aung San Suu Kyi for most of the following two decades.
It boycotted parliamentary elections in 2010 because of election laws it said were unfair, but won 43 seats in by-elections in 2012, including one for Aung San Suu Kyi, as a reform process introduced by the nominally civilian administration of President Thein Sein gathered pace.
Nearly 900 delegates have been selected to attend the congress in Rangoon, at which they will elect new party leaders, with Aung San Suu Kyi widely expected to retain the top post.
The NLD's diverse membership have historically been united by their opposition to Burma's military rulers and their loyalty to Aung San Suu Kyi, says the BBC's Jonah Fisher in Rangoon.
But democratic reforms mean the NLD is now part of Burma's political system and this ageing and sometimes chaotic movement is badly in need of an overhaul, our correspondent adds.
Senior member Han Tha Myint said he hoped some new faces would also be brought in. "We didn't get this chance in the past," he told Reuters.
Analysts say the party has also struggled to find a new direction and to effectively set out its policies ahead of parliamentary elections due in 2015. It also need to show it has the capacity to govern, they say.
Burma's opposition National League for Democracy (NLD) is holding its first ever congress, at which it will elect party leaders for the first time.