The black-necked crane Grus nigricollis is the only alpine crane species, and its distribution is restricted to the Tibetan-Qinghai Plateau and the adjacent high altitude areas of China, Bhutan, and India. With a world population of 10000-10200 birds, this species is classified as vulnerable under the IUCN Red List Categories. The Ruoergai wetland is situated at the headwaters of the Yellow River on the northeastern edge of the Tibetan Plateau in western China, and it is one of the largest wetlands in China. With over 2,000 cranes, the marsh is the most important breeding area for the black-necked crane in the world. The Ruoergai National Nature Reserve, a wetland site of international importance and accounting for only 8.3% of the Ruoergai wetland, hosts over 1/3 of the total black-necked cranes population breeding in the Ruoergai wetland. Nest building is a significant strategy for many bird species to secure breeding success, and many species spend considerable time and energy in nest building activities. For birds, nest building is one of the key indicators for parental investment during the breeding period, and the duration of nest building greatly reflects the birds' adaptation to the environment. To enhance knowledge of the black-necked crane breeding biology, and better understand its ecological adaption and conservation strategy, we studied the behavior at the Ruoergai National Nature Reserve from March-July in 2013 and 2014. The specific objectives of this study were to determine the duration of nest building of this species and its relation to:(1) nest type; (2) starting date of nest building; (3) nest size; and (4) nesting habitat. All potentially nesting black-necked cranes pairs in the study area were located with field surveys and data from previous observations. We checked these pairs every day, accessing the site with field vehicles. Once arrived in an area where a pair of cranes was sighted, we watched from a distance with a spotting scope, and recorded the date, time, and nest building activity. Immediately after the hatching of crane chicks, we walked to the nest and recorded the nest type, size of the nest (length, width, and height), and nesting habitat. There were four types of nests:grass mound, natural island, dirt mound, and floating grass mound. Nesting habitats included lake, river, and shallow water marsh. A total of 55 nests of black-necked cranes were studied. From these 55, 41 were grass mounds, five were natural islands, five were dirt mounds, and four were floating grass mounds. Twenty-five nests were located in swampy habitats, 17 in lake habitats, and 13 near rivers. The cranes spent (6.7±9.3) days on nest building, ranging from 0.5 to 40 days. From the four types of nests built or used by the cranes, nest building period was from longest to shortest:dirt mound > grass mound > floating grass mound > natural island. Nest building time and nest types correlated significantly (r=0.728). As for habitat type, nest building took the longest time in the lake habitat. Most nests were built in April, when cranes also spent more days building their nests, than in May and June. Finally, the larger the nest, the longer it took the cranes to build it.