The mass attenuation coefficients (pip) of some tissues such as muscle (ICRU-44), adipose (ICRP) and blood (Whole) and tissue equivalents such as soft tissue model (H63C6O28N) and water have been investigated using Geant4 simulation tool kit. Appreciable variations have been noted for mu/rho values by changing the photon energy for the studied tissues. The simulated mu/rho have been compared with experimental data available in the literature and theoretical XCOM results in the energy region 1 keV-100 GeV, and good agreement has been observed. Also, mass attenuation coefficients relative to water have been calculated in the entire energy region to evaluate the water equivalence of the studied tissues. It is shown that a maximum difference of 8.8 % between water and mentioned soft tissue is observed at 8 keV and soft tissue is found to be a good tissue equivalent for blood and muscle tissue.