It has been found for the first time that excess argon exists in plagioclase p henocryst in Tengchong Heikongshan, Dayingshan and Maan shan volca nics. The KAr ages obtained from the matrix are younger than those of the whol e rocks, which demonstrates that the published volcanic ages belonging to the Quat ernary are older than their forming time because of the excess argon. Combined w ith the Useries dating in the Maanshan volcano, it was considered that the end time of the latest volcanic eruption in the Tengchong area was in Holocene.