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Located in the transitional region between the Sichuan Basin (SCB) and Qinghai-Tibetan Plateau (QTP), the West China Rain Zone (WCRZ) is a large-scale ecotone and partially belongs to the Southwest China Mountains, which is one of the world's 34 biodiversity hotspots. Using observation data from national air quality stations and our own monitoring data, we investigated the risk from O 3 to vegetation and the major source-region of O 3 for two UNESCO (i.e., United Nations Educational, Scientific and Cultural Organization) world heritage properties (Mt. Qingcheng and Mt. Emei) and one city (Ya'an) in the WCRZ. The results show that the annual mean maximum daily 8-h average (MDA8) O 3 concentration in Mt. Qingcheng (54 ppb) was higher than that in the adjacent SCB cities (38-48 ppb). The acute and chronic risk levels from O 3 to vegetation were also higher in Mt. Qingcheng than at all the other sites. The mean MDA8 O 3 concentrations and the O 3 risk levels to vegetation in Mt. Emei and Ya'an fell in the range of that at the SCB and QTP cities. However, O 3 exposures at all the WCRZ, SCB, and QTP sites exceeded the empirical critical loads for natural ecosystems, forest trees, and highly O 3 -sensitive plants. The SCB was identified as the largest source-region of O 3 for Mt. Qingcheng and Mt. Emei but other Chinese regions and northern India also had considerable contributions. To protect biodiversity and ecosystem services, there is a need to further systematically study O 3 and its ecological impacts for the entire WCRZ., Competing Interests: Declaration of competing interest ☑ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)