Concrete is taken as three-phase composites consisting of aggregate, interface and mortar matrix in the paper. The tensile strength and elastic modulus of each phase material are assumed to obey Weibull distribution. A few random aggregate and random parameter models are generated by Monte-Carlo method. Tensile failure of micro element is described by tension damage model, in which the stress will be reduced linearly after the stress reaches the maximum tensile strength. The results show that more cracks of lower homogenous specimens are observed than those of higher homogenous specimens. The phenomenon of discontinuous cracking is observed in the former, while strain localization and brittle behavior is observed in the latter. The ultimate load obtained in the numerical simulation agrees well with that of the test.