Willa Cather, an early twentieth-century American novelist, is famous for her use of rich natural imagery, and also for her beautiful landscape description. She is known as an American nature writer. Through an ecocritical lens, Cather’s environmental ethical view is multivalent. Sometimes she treats humans as nature’s owners while at other times she sees the human species as part of the natural system. Ecocriticism draws people’s attention to the tension between humanity and nature and urges a change of worldviews from the dominant anthropocentric perceptives to non-anthropocentric ones as the proper environmental ethic. By studying Cather from the standpoint of environmental ethics, we can define her place in the field of ecocritical concern. This thesis focuses on the imagery of nature and the portrayal of the human-nature relationship in Willa Cather’s O Pioneers! and My Antonia in order to see Cather’s environmental ethical worldviews. First, analyzing Cather’s portrayal of nature from the conventional literary mode of pastoralism can help to explore how she looks at nature anthropocentrically. In addition to the analysis of her representation of the natural world, her valuation of actual environmental practices is also important in understanding Cather’s environmental concerns. Relating her to the ongoing environmental debate between biocentric preservationism and anthropocentric conservationism of her time reveals her positionality or positionalities in the environmental discussion. On one hand, she provides a biocentric preservationist view, a view consistent with that of modem environmentalists, while most of the time, she seems to prefer an anthropocentric conservationist practice, which is opposite to that of preservationism. However, Cather’s position in ecocriticism becomes even more complicated when I relate her to two other environmental ethics, theocentrism and ecocentrism, since those ethics, which are believed to be more appropriate for an environmental approach, only further reveal the tension between her anthropocentric and non-anthropocentric tendencies. The study of Cather in terms of environmental ethics also reveals the conundrum of humanity in the environmental discussion. Paradoxically, ecocriticism asks us to live with nature in the most unnatural way: we see ourselves no different from nature, but practically we can only live harmoniously with nature through our unique feature of self-awareness and self-regulation. Willa Cather, who shares her culture’s ambivalence between human-centered and nature-centered beliefs, is the clearest example of conundrum.