Abstract: Coal is a special kind of rock. There are plenty of fissures and weak faces distributed randomly over the coal samples, resulting in great difference of mechanical behavior of coal samples with different sizes. Based on a group of experimental results of coal samples with different diameters (D=61, 101, 146 and 300 mm) under conventional triaxial compression obtained by Medhurst and Brown (1998), the confirmation methods for strength parameters of coal samples with different diameters are discussed. The results show that the strength of coal samples is the function of the confining pressure and the diameter. With the increase of the diameter, the elastic modulus of coal samples decreases by means of a negative power function, nearly independent of the confining pressure. The Poisson’s ratio of coal samples is not related to the diameter and confining pressure and is 0.21~0.25. By considering the relation between the generalized Hoek-Brown criterion parameters and the diameter, the strength of coal samples with different diameters and confining pressures can be predicted approximately in the range of 0≤σ3≤10 MPa and 61 mm≤D≤300 mm. In accordance with the generalized Hoek-Brown criterion parameters, the cohesion and internal friction angle of coal samples with different diameters are obtained by the tangent method theory, which has the shortage of good approximation in a certain test point but bad approximation on the whole. For a kind of rock evaluated as a generalized Hoek-Brown material, a new evaluation criterion is put forward by adopting the optimal approximation polynomial theory (optimal approximation first equation) in functional analysis. It is employed to confirm more precisely the cohesion and internal friction angle of coal samples with different diameters. Finally, the effect of the diameter on the cohesion and internal friction angle of coal samples is investigated. The research results provide the reference value for the choice of the cohesion and internal friction angle of rock samples in the process of geo-engineering design and numerical simulation.