This work explored the way to improve hot modulus of rupture (HMOR) and refractoriness under load (RUL) by adding mild-calcined coal gangue (MCG) in Al2O3-SiO2 ultra low cement (ULC) castables, making use of the in-situ effect of the MCG during heating-up. The influence of respective additions of 5%, 10% and 15% of the MCG powders calcined at 700 ℃ was investigated on HMOR at 1 400 ℃ and RUL of the castables. With increased addition of the MCG, HMOR and RUL become significantly enhanced. At 10% of the MCG addition, HMOR reaches 3 MPa, as compared to 0.3 MPa in the case of no MCG addition. RUL of the specimens dried at 110 ℃ for 24 h can be increased by some 270 ℃ with 10% of the MCG addition. RUL of the specimens preheated at 1 500 ℃ for 3 h maintains the growth trend with the MCG addition increasing. The microstructure of the heated castable samples was investigated by means of SEM. The in-situ formed needle-like and interlaced mullite in the matrix is contributive to the improvement.