Aluminum alloy (Al-alloy) thin-walled (D/t > 20, diameter D, wall thickness t) bent tubes have attracted increasing applications in many industries with mass quantities and diverse specifications due to satisfying high strength to weigh ratio requirements of product manufacturing. However, due to nonlinear nature of bending with coupling effects of multiple factors, the similarity theory seems not applicable and there occurs a challenge for efficient and reliable evaluation of the bending formability of thin-walled tube with various bending specifications. Considering the unequal deformation and three major instabilities, the bending formability of thin-walled Al-alloy tube in changing tube sizes such as D and t are clarified via both the analytical and FE modeling/ simulations. The experiments of rotary draw bending are conducted to validate the theoretical models and further confirm ’size effect’ related bending formability. The major results show that (1) The anti-wrinkling capability of tube decreases with the larger D and smaller t, and the effect significance of t is larger than that of D even under rigid supports; (2) The wall thinning increases with the larger D and smaller t, and this tendency becomes much more obvious under rigid supports; (3) The cross-section deformation increases with the larger D and smaller t according to the analytical model obtained intrinsic relationship, while this tendency becomes opposite due to the nonlinear role of mandrel die; (4) The size factor D/t can be used as a nondimensional index to evaluate both the bending formability regarding the wall thinning and cross-section deformation. Bent tubes have attracted growth increased applications in many industries with mass quantities and diverse specifications due to satisfying high strength to weigh ratio requirements of product However, due to due to nonlinear nature of bending with coupling effects of multiple factors, the similarity theory seems not applicable and there occurs a challenge for efficient and reliable evaluation of the bending formability of thin-walled tube with various bending specifications. Considering the unequal deformation and three major instabilities, the bending formability of thin-walled Al-alloy tube in changing tube sizes such as D and t are clarified via both the analytical and FE modeling / simulations. The experiments of rotary draw bending are conducted to validate the theoretical models and further confirm ’size effect’ related bending formability. The major results show that (1) The anti-wrinkling capabili ty of tube decreases with the larger D and smaller t, and the effect significance of t is larger than that of D even under rigid supports; (2) The wall thinning increases with the larger D and smaller t, and this disadvantage becomes much more (3) The cross-section deformation increases with the larger D and smaller t according to the analytical model obtained intrinsic relationship, while this tendency becomes opposite due to the nonlinear role of the mandrel die; (4) The size factor D / t can be used as a nondimensional index to evaluate both the bending formability regarding the wall thinning and cross-section deformation.