Medical teams battled to reanimate a 13-year-old girl and an adult man at the showground in Xanten, before rushing them to hospital.
A storm had hit the festival where more than 20,000 visitors had showed up to see more than 400 people dressed as Roman soldiers, gladiators, craftsmen and traders.
Many of the visitors took shelter under trees, who of which were struck by lightning, injuring people standing nearby.
More than 70 emergency staff were called to the site, and used a helicopter as well as ambulances to take the injured to hospitals.
The festival, which bills itself as the biggest Romans gathering in Europe, was due to continue on Sunday, was cancelled.