The black-necked crane Grus nigricollis is the only alpine crane species, whose distribution is restricted to the Tibetan-Qinghai Plateau and the adjacent high altitude areas of China, Bhutan, Pakistan, and India. Study on nest type and size is useful for understanding the life history, evolution and adaptation of birds. A survey was conducted on nest size and the underlying regulatory factors of Black-necked Cranes from 25 March to 31 July in 2013 and 2014 at Ruoergai Internationally Important Wetland and its surrounding area, covering 83 nests. Four types of nests were found in the area, i.e., natural island nest, grass mound nest, dirt mound nest and floating grass nest. The nest length, nest width, nest height and nest volume among the four types of nests all were significantly different in the order: floating grass nests >dirt mound nests >grass mound nests >natural island nests, indicating that nest size was related to nest type. Among the three kinds of nest-site habitats, there were no significant differences in nest length and nest width, but differences were found in nest height and nest volume in the order: lake nests >river nests >swamp nests, indicating that nest size of Black-necked Cranes was also related to nest-site habitat. Nest length, nest width, nest height and nest volume were all found to be the greatest in April while the smallest in June with medium size in May, implying that nest size tended to decrease with time over a year.