Competition and coexistence are two common phenomena occurred in natural ecosystems. The classic resource competition model, i.e., the Monod model, supports the competitive exclusion principle, which suggests that the number of species in an ecosystem cannot exceed the number of limiting resources. Hence, this model fails to explain the coexistence of species with high diversity in plant communities. A resource competition model with self-inhibition is developed in this study based on the concept that the species growth rate may reach its limit as the body size or the number of individuals of the species gradually increases toward its limit. The essential difference from the Monod model is that in the new model, the critical resource availability increases as the population density rises to achieve a balance among various species. The results of mathematical analysis and numerical simulation show that the model allows for the coexistence of multiple species in which the dominant species change occurs near the gradient of the resource supply rate.