Yuan Dynasty chose Tibetan Buddhism thanks to the following causes: widely-spread influence of Tibetan Buddhism in Northwest China, interaction between Mongolians and Tibetans, Tibetan Buddhism conforming to society and culture of that time, close relationship between Mongolian rulers and Tibetan Buddhists, prosperity of Tibetan Buddhism in West Xia, Tibetan Buddhists' active missionary work to the Mongolian royals, and the characteristics of Tibetan Buddhism.