Le Pen, the RN itself, and 24 others - party officials, employees, former MPs and parliamentary assistants - are accused of using money destined for work at the European Parliament to pay staff who were working for their party, which at the time was called the National Front.
Le Pen and the other accused say the way they used the money was legitimate and the allegations show too narrow a definition of what a parliamentary assistant does.
"I approach this trial with great peace of mind," Le Pen told reporters on Monday as she arrived at the criminal tribunal in Paris.
The trial could potentially be damaging for Le Pen, who, if found guilty, risks being barred from public office for several years for accusations she denies.
But if it clears her of wrongdoing, the trial, which comes almost a decade after initial investigations started, could also help Le Pen further polish her image and that of her party before the 2027 presidential election.
Le Pen, who lost to Emmanuel Macron in the second round of France's presidential election in 2017 and 2022, is widely seen as a frontrunner in 2027.
The European Parliament had estimated the damage at 3.5 million euros ($A5.7 million), its lawyer Patrick Maisonneuve said.
EU MPs are allocated funds to cover expenses, including their assistants, but are not meant to use them to cross-fund party activities.
Le Pen and her party, which argues for "France first" policies on issues ranging from immigration, energy markets and agriculture, reject the allegations.
RN MP and party spokesman Laurent Jacobelli said last week that what was at stake was "a different understanding, as a French party, of what an assistant role is, compared with the European Parliament's understanding".
Le Pen faces charges both for her role as party leader and as an EU MP at the time, for allegedly hiring fictitious assistants herself.
If found guilty, Le Pen and other defendants could face a potential jail sentence of up to 10 years and a one million-euro fine.
Those like Le Pen who were elected officials at the time of the alleged offences also risk being barred from public office for up to 10 years.Â
Those who were not elected officials could be barred for up to five years.
Le Pen's father, former National Front leader Jean-Marie Le Pen, 96, is also among the accused but, at the start of the proceedings the judge said that, for health reasons, he and one another accused were not fit to stand trial and would be judged separately at a later stage.
The trial will last until November 27.
