The bomb attack occurred on a rural road in tambon Pakprue of Mae Lan district about 12.30pm, according to the reports.
Police investigators reported that the rangers from Company 22 were patrolling along the road on a vehicle when a home-made-bomb hidden in a motorcycle parked on the roadside was detonated.
Ranger Kowit Jitbut, 29, Capt Sompran Ponlarp, 52, Corp Sonthaya Kerdbut, 28, and Ranger Krik Chaikamain, 38, were injured in the bomb blast.
They were taken to Mae Lan hospital, where Ranger Kowit was later pronounced dead.
Police and a explosives ordnance disposal unit were inspecting the scene.