The grenades were believed to be live and could have exploded with pressure or heat.
Muang Ubon Ratchathani Police Station investigator Lt-Colonel Sittichok Piboonsaksophol said Kamol Boonme, 30, was using a back?hoe on the roadside when he found the rusty grenades, which were 60mm, 81mm and 120mm in size.
The grenades were sent to an army camp and blown up.