Hoek-Brown strength criterion has been used for analysis of a wide range of problems in rock mechanics and rock engineering. For intact rock, the criterion is mainly controlled by the strength parameter mi which is empirical and developed by trial and error. By representing intact rock as a densely packed cemented granular material, its mechanical behavior can be modeled using the particle flow modeling. In this paper, the three dimensional particle flow code PFC3D is used to conduct numerical tests on intact rock and investigate the effect of micro-parameters on the strength parameter mi. The micro-parameters which may affect the strength parameter mi, such as particle size, contact model type, the normal and shear strength are studied respectively. The results show that the particle size only has minor influence on the strength parameter mi and its effect can be simply ignored. The contact model type and the contact normal to shear strength ratio, however, both have major effect on the strength parameter mi. The parallel-bond model gives higher mi values than the contact-bond model. When the normal to shear strength ratio is smaller than 1, higher shear strength leads to large increase of parameter mi.