Although fat-tail has long been considered as an energy source for working, frying materials and their increased durability and also as a valuable source of energy storage for nomadic people, nowadays due to machinations of life, fat-tail and especially, its large size is challenging. Fat-tail production conflicts with the national benefits, economy of farmer and public health. The fact is that the amount of food needed for fat-tail sheep is high and the consumer’s pattern has been changing from fat to lean meat consumption. Therefore, actions have been performed to reduce or eliminate fat-tail using physical cut by rubbers rings or crossing the fat-tailed breeds with Zel sheep. The results of previous research have indicated that the size of fat-tail should be reduced through selection genetic within the fat-tailed breeds. For this purpose, it is necessary to record the animal's fat. To measure the exact weight of sheep fat-tail slaughtering is necessary. In this situation, the animal cannot be selected in the breeding program, so using the regression models to estimate the fat-tail weight in live animals is necessary. So far, two studies have been undertaken to predict fat-tail weight using its different dimensions on Lori-Bakhtiari sheep. It seems that the best way to predict fat-tail weight is to find its volume and then finding a mathematical relationship between size and weight. This could lead to a reduction in the number of independent variables and combine them to estimate fat-tail weight accurately. The findings of the present study revealed that coefficient determination of the model was 0.96 which was superior about 0.14-0.34 as compared to the other measuring methods.