The handwriting of 39-year-old V Irulappan, hailing from Kovilpatti, matched with that found in the three letters received by plant authorities, police said.
The letters were signed in the name of 'Makkal Podhunala Kuzhu' (Public Welfare Committee of the People).
Police were able to trace Irulappan after it was found that the letters had the post mark of Kovilpatti area.
They took handwriting samples of four suspects and matched them with the contents of the letters at Regional Forensic Sciences Laboratory in Madurai.
Irulappan was arrested on Sunday.
Leaders of People's Movement Against Nuclear Energy, spearheading the stir against the nuclear power project, had said it has no role in the threat letters.