Samples in a gasketed diamond anvil cell (DAC) are usually subjected to a uniaxial stress at high pressure due to the geometric structure of the DAC, namely a maximum stress along the cell loading axis and a minimum stress in the radial direction.Using conventional DAC in situ high pressure X-ray diffraction method， the measured compression curves often lie above the hydrostatic compression curves because of the effect of uniaxial stress field.Using the technique of DAC radial X-ray diffraction and the lattice strain theory, through the recent experiments with W, diamond and B6O samples，we discussed how the unixalxial stress filed, sample strength, and the sample difference between the sample material and pressure standard material affect the determination of equation of state.