The effective atomic number (Z(eff)) and effective electron density (N-eff) of eight heavy metal oxide (HMO) glasses have been determined using the Monte Carlo simulation code MCNP for the energy range of 10 keV-10 MeV. The interpolation method was employed to extract Z(eff) and N-eff values from the simulation and that calculated with the help of XCOM program. Comparisons are also made with predictions from the Auto-Z(eff) software in the same energy region. Wherever possible, the simulated values of Z(eff) and N-eff are compared with experimental data. In general, a very good agreement was noticed. It was found that the Z(eff) and N-eff vary with photon energy and do not have extended intermediate regions where Compton scattering is truly dominating; only dips slightly above similar to 1.5 MeV were recorded. Z(eff) and N-eff are found to increase with PbO and Bi2O3 contents. It was found that the Z(eff) value rather than the N-eff value is a better indicator for PbO and/or Bi2O3 contents.