Abstract: The aim of this study is to accurately estimate the weight of polled yak by its body size，and to subdivide the effects of body parts’ sizes on the weight. In the experiment，247 polled yaks aging 8 to 10 months，155 male yaks and 92 female yaks，were randomly selected as objects of the study. Four body size data of body length，standing height，chest circumference，and cannon bone circumference，as well as a yak’s weight were measured. Multiple linear regression equations were established by stepwise multiple linear regression. Then the direct and indirect effects of each body size on weight were calculated by path analysis. The results suggested that the correlation between chest circumference and cannon circumference of male yak did not reach significant difference（P >0.05），and the phenotypic correlations among other traits reached extremely significant difference（P < 0.01）. There was no significant difference between the estimated values and the observed values of the weight. The direct effect of chest circumference on the weight was greater than the effects of the other body parts’ sizes，and greater than the indirect effect of chest circumference through the other body parts’ sizes. In conclusion，the multiple linear regression equations can be used for selective breeding practice in polled yak，and the path analysis suggests that the dominant factor of impacting polled yak weight was the chest circumference.

Key words: polled yak, body size, weight, multiple linear regression, path analysis