A flexible potential including both intra - and inter - molecular potential was used to calculate the lattice properties of several secondary explosives such as nitramine crystals of HMX(1,3,5,7- tetranitro - 1,3,5,7 - tetraazacycrooctane). RDX (1,3,5 - trinitro - 1,3,5 - triazacycrohexane) and DMN (N,N-Dimethylnitramine), and non - nitramine crystals of PETN (Pentaerythritol tetranitrate), ANTA(3 - Amino - 5 - nitro - 1,2,4-triazole). NB (Nitrobenzene) and NM (Nitromethane). The intramolecular part of the potential contains bond stretching, angle bending, out - of - plane bending, torsional, and non - bonded motion terms. Parameters in these terms were taken from literatures and refitted to reproduce the experimental crystal structures and infrared spectrs of these explosives. The Buckingham exp - 6 function with Coulombic interactions was used for intermolecular potential of HMX, RDX, NB, DMN and NM, whereas the Lennard-Jones 6-12 potential with Coulombic interactions was applied for intermolecular interactions in PETN and ANTA (plus hydrogen bonding potential). It is confirmed that the flexible potentials could accurately predict lattice parameters and lattice energies of these explosives. In addition, the potentials could also predict the bulk modulus and its pressure derivatives. Calculated bulk modulus of NM had a deviation less than 4.5 % compared with the experimental data at pressure region of P ¡å 3.0 GPa. Calculated elastic constants, elastic stiffness, averaged Young's modulus and Poisson ratio of RDX were in agreement with the experimental data.