Functional diversity of a plant community, which is based on plant traits, has been proposed as a key component predicting ecosystem function. Knowledge about how plant functional traits and functional diversity change along with the different slope aspects at the community level is important for understanding the formation of plant communities under distinct slope aspects. However, to date, previous studies on plant communities in sunny and cloudy slope environments have mainly been descriptive. To our knowledge, no research has dealt with the relationship between the slope aspect and community assembly based on plant functional diversity (i.e., functional richness, evenness and divergence). In this study, we aimed to uncover the relationship between slope aspects and plant functional traits. Three sampling transects (1 m × 5 m) were established on each of four sites facing north, south, east, or west, in Malan Mountain, Loess Plateau. Each of the sampling transects were evenly divided into fifteen 1 m × 1 m plots, and a plot survey was conducted. We measured plant height (H) and specific leaf area (SLA), and then calculated three types of functional diversity indices (i.e., functional richness, evenness, and divergence), which included three one-dimensional indices based on single traits (i.e., FRci, FRO, FDvar) and three multi-dimensional indices based on two traits (i.e., FRci, FEve, FD_Q). A comparison of the changing pattern of plant functional traits and the three types of functional diversity was conducted among the four slope aspects (the cloudy, sunny, semi-cloudy, and semi-sunny slope). Our results indicated that: (1) the value of H and SLA on the cloudy slope was greater than the others; (2) there was no significant difference among the four slope aspects in one-dimensional functional richness indices of H and SLA, while the multi- dimensional functional richness indices of plants on the cloudy slope and the semi-cloudy slope were greater than those of the plants on the sunny slope; (3) on the cloudy slope, the one-dimensional functional evenness of H was significantly greater compared to those on the semi-sunny slope, while that of SLA was not significantly influenced by the slope aspect; the multi-dimensional functional evenness indices of plants on the cloudy slope was greater than those of the plants on semi-sunny slope; (4) the one-dimensional functional divergence of H on the cloudy and semi-cloudy slope were significantly greater than those on the semi-sunny and sunny slope, while that of SLA on the semi-cloudy slope was significantly greater than those on the sunny slope; the multi-dimensional functional divergence index and Rao's quadratic entropy of plants was significantly greater on the semi-cloudy slope compared with those on the sunny slope. Our results indicated that for plant traits, more niche space was occupied, more resources were sufficiently exploited, lower competition intensity occured, and a high level of niche differentiation existed on the cloudy and semi-cloudy slopes. By contrast, because of the limitation of all kinds of soil conditions and environmental factors, plant traits occupied less niche space on the sunny and semi-sunny slope, resources were exploited less sufficiently, and competition for resources tended to be stronger. At the community-level, the changing pattern of plant functional traits and functional diversity along the four different slope aspects suggests the existence of an environmental fitter effect on functional traits under the process of community assembly in the Loess region. These findings would have significant practical implications for species selection and vegetation rehabilitation layout planning in the hilly area of the Loess Plateau.