The grenade was thrown last night at the Borgarah tea garden manager's office, in which the manager Rakesh Kureshia and assistant manager Ramakant Tiwari were injured.
"A tea trader Kamal Chand Jain, who was present in the office at that time, died on the way to Guwahati," a police officer said.
As per witnesses, four persons on two motorcycles entered the garden from the northern side and threw the grenade at the manager's office, where the three persons were sitting.
Further investigation is on to nab the culprits.