The purpose of hydraulic fracturing is to improve the gas permeability of a coal seam by the high-pressure injection of fracturing fluid into cracks. This paper simulates the hydraulic fracturing of a coal seam, investigates relevant parameters and analyzes the connection between macroscopic mechanical parameters and mesoscopic mechanical parameters based on two-dimensional particle flow code (PFC2D). Furthermore, the influence of macroscopic mechanical properties on the initiation and size of cracks is studied based on various combinations of particle flow calculations. Empirical formulae for the breakdown pressure and fracture radius are derived. Moreover, the effect of the injection parameters on crack propagation is computed and analyzed, after which the relevant empirical formula is proposed. Finally, numerical simulation of the working face N3704 at Yuyang Coal Mine (YCM) is conducted, and the comparison of results from simulation, empirical formulae and field observation is investigated. The research findings of this paper may provide a reference for selecting injection parameters and forecasting the effect in practical hydraulic fracturing applications. (C) 2013 Elsevier B.V. All rights reserved.