Objective To study the body mass of 13 tribal Mongolians, and to reveal the relationship between body mass and longitude, latitude, annual mean temperature and age of Mongolian. Methods Using the method of random sampling, following the principle of informed consent, the body mass of 13 ethnic Mongolian groups 4410 people(male 1917, female 2493) and the physical quality related to body mass were measured. Results The average body mass of the 13 ethnic groups in Mongolia was (57.3±8.1)kg to (81.6±12.7)kg. The 1st to 3rd places of the average body mass ranked from the largest to the smallest were the Ejina Torgouts, the Chahar of Xinjiang, and the Ordos, the 11th to 13th places were the Kazuo Mongolian, the Fuxin Mongolian, and the Yunnan Mongolian. The average body mass of women in 13 ethnic groups was between (51.6±7.6)kg to (67.7±12.6)kg. According to the average size of the body mass, the first to third places were Ejina Torgouts, Barag, and Xinjiang Torgouts, the 11th to 13th places were the Kazuo Mongolian, the Goerros, and the Yunnan Mongolian. Differences among 13 ethnic groups, age groups, and body mass were statistically significant. The fat mass values of women of all ethnic groups were greater than those of men. The fat free mass values were greater for male than female. The body mass and fat free mass of Mongolian males and females were negatively correlated with longitude and annual mean temperat re, both of which were significantly positively correlated with latitude. There was no significant correlation between male body mass and age, while females were significantly positively correlated. Fat mass of males and females was positively correlated with latitude and age, but not with longitude, and negatively correlated with annual mean temperature. Mongolian body mass was greater than the other 29 ethnic groups in China. Conclusion Mongolians have higher body mass values.