This paper present a modification of selected strength criteria for coal and rocks containing weakness planes._x000D_
The three strength criteria, namely, Murrell (1965), Bieniawski (1974a, 1983) and Hoek and Brown (1980) have been assessed for coal (containing weakness planes) by using triaxial strength tests._x000D_
Triaxial test results indicated that the strength of coal specimens depends on the weakness plane inclination with respect to the principal stress direction._x000D_
However it is shown that with increasing confining pressure, the effect of weakness planes inclination on the strength of coal specimens is reduced._x000D_
The constant parameters in the three failure criteria for coal are determined for each weakness plane inclination by using the Datafit computer program (nonlinear regression analysis) and the results are compared with other rock types.