Forest officials in the north eastern area of India discovered the bodies of the group of 18 clustered together in two groups.
They suspect they died during a recent storm in the Nagaon district of Assam state.
Assam Chief Minister Himanta Biswa Sarma said in a statement that he was concerned about the deaths of such a 'huge number of elephants'.
Amit Sahay, from the state's forest department, said: 'Four elephant carcasses were found at one spot and 14 others at another spot.
'The preliminary investigation revealed that the elephants could have been killed by lightning.
'Top officials of the forest department and veterinary doctors have been rushed to the spot.'
Assam's Forest Minister Parimal Suklabaidya tweeted: 'Deeply pained by the death of 18 elephants last night due to massive thunderstorm under Kothiatoli Range in Nagaon.'
India is home to nearly 30,000 elephants, around 60 per cent of the wild Asian elephant population.
Wild elephants in the state have previously been victims of poaching, train-related accidents, poisoning and even electrocution, according to local media.