Bomb disposal officers were called to Wat Yarng Wanaram at Moo 5 in tambon Ban Tha Ngarm of Wat Bot district on Friday afternoon to move and destroy the old shell, which was believed to be still functional.
Abbot Phra Arthikarn Uttamo said a layman had given the device as a "token" to the temple and its late former abbot about 20 years ago. It had been kept in a display cabinet in the sermon hall ever since.
Phra Arthikarn said this week's deadly explosion at a junkyard in Bangkok had stirred worries about the safety of monks and temple visitors, so he asked the authorities to examine the shell and demolish it.
Pol Capt Jakkrit Kwangkaew, the chief of Phitsanulok's Explosive Ordnance Disposal (EDO) unit, said the device was an 81mm mortar round, adding that while it was old it remained functional.
His team brought the device to be detonated on open ground in the tambon. The explosion left a 50-centimetre-deep crater in the ground, he said.