Four of the victims remain in hospital, with two people seriously injured, according to police reports.
The accident happened shortly after 7pm at a tent where Asian food was being served, Lucerne police said.
The injuries were caused when a flame flared from a gas bottle used for cooking.
Al the victims were employees of the food stall, police said.
The cause of the fire is under investigation.
The incident did not affect concerts at the 20th annual music festival, which is held at various venues around Lake Lucerne.
The nine-day festival, running until July 28th, feature rock bands such as The Kaiser Chiefs and The Kills.
Elsewhere in Switzerland, the 37th Paleo music festival in Nyon (canton Vaud) wrapped up on Sunday after attracting 230,000 spectators over the course of six days.
The festival’s performers included Sting, The Cure, Franz Ferdinand and David Guetta.