Abstract: Coal has also scale-dependent mechanical properties like many other rock materials. Based on the Weibull theory and the assumption that strength is randomly distributed for brittle materials, a scale constitutive model in uniaxial compression for coal samples with the effect of interfacial friction is developed according to the theory of damage mechanics, and then the model is verified by uniaxial compressive experiments. Results show that the ultimate compression strength of coal samples obviously decreases as the aspect ratio increases, and the interfacial friction becomes the main reason for the differences of the results between Weibull theory and experiments. While the results obtained from the modified model agree better with experiments ones, it can reflect the law of changeable uniaxial compressive strength with the coal sample size and describe almost the whole process of uniaxial loading.