Ascertaining the origin of zircons from the ultrabasic- basic rocks is the key to determine the rock- forming ages of them. A large number of ultrabasic- basic volcanic and intrusive rocks exposed in the metamorphic strata in western Jiangnan orogenic belt (including northern Guangxi, eastern Guizhou and southern Hunan). Previous studies had suggested that these ultrabasic- basic rocks were formed in Neoproterozoic relating to the Sibao orogeny and post- orogenic rift development. Re- examine the previous dating data published in recent years, along with new dating data in this text, we found that a few Paleozoic- Cenozoic zircons widespread exist in these “Neoproterozoic” ultrabasic- basic rocks. Based on the analysis of zircon morphology and cathodeluminescence (CL) characteristics, we conclude that the U- Pb ages of these young zircons cannot be completely explained by later hydrothermal transformation or mixing during sample selection. This paper is to discuss the diagenetic ages of ultrabasic- basic rocks. If the rock- forming ages of ultrabasic- basic rocks in western Jiangnan Orogen were wrongly explained in previous studies, the tectonic evolution history of the western Jiangnan orogenic belt need further discussion.