In this paper,we report the assessment of the effect of the three-dimensional(3D)density heterogeneity in the mantle on Earth orientation parameters(EOP)(i.e.,the polar motion,or PM,and the length of day,or LOD) in the tidal frequencies.The 3D mantle density model is estimated based upon a global S-wave velocity tomography model(S16U6L8) and the mineralogical knowledge derived from laboratory experiment.The lateral density variation is referenced against the preliminary reference earth model(PREM).Using this approach the effects of the heterogeneous mantle density variation in all three tidal frequencies(zonal long periods,tesseral diurnal,and sectorial semidiurnal)are estimated in both PM and LOD.When compared with mass or density perturbations originated on the Earth’s surface such as the oceanic and barometric changes,the heterogeneous mantle contributes less than 10% of the total variation in PM and LOD in tidal frequencies.However,this is the gap that has not been explained to close the gap of the observation and modeling in PM and LOD.By computing the PM and LOD caused by 3D heterogeneity of the mantle during the period of continuous space geodetic measurement campaigns(e.g.,CONT94) and the contribution from ocean tides as predicted by tide models derived from satellite altimetry observations(e.g.,TOPEX/Poseidon) in the same period,we got the lump-sum values of PM and LOD.The computed total effects and the observed PM and LOD are generally agree with each other.In another word,the difference of the observed PM and LOD and the model only considering ocean tides,at all tidal frequencies(long periods,diurnals,and semidiurnals) contains the contributions of the lateral density heterogeneity of the mantle.Study of the effect of mantle density heterogeneity effect on torque-free Earth rotation may provide useful constraints to construct the reference earth model(REM),which is the next major objective in global geophysics research beyond PREM. In this paper, we report the assessment of the effect of the three-dimensional (3D) density heterogeneity in the mantle on Earth orientation parameters (EOP) (ie, the polar motion, or PM, and the length of day, or LOD) in the tidal frequencies. The 3D mantle density model is estimated based upon a global S-wave velocity tomography model (S16U6L8) and the mineralogical knowledge derived from laboratory experiment. The lateral density variation is referenced by the preliminary reference earth model (PREM). Using this approach the effects of the heterogeneous mantle density variation in all three tidal frequencies (zonal long periods, tesseral diurnal, and sectorial semidiurnal) are estimated in both PM and LOD. Wen compared with mass or density perturbations originated on the Earth’s surface such as the oceanic and barometric changes, the heterogeneous mantle contributes less than 10% of the total variation in PM and LOD in tidal frequencies. However, this is the gap that has not been explained to clo se the gap of the observation and modeling in PM and LOD.By computing the PM and LOD caused by 3D heterogeneity of the mantle during the period of continuous space geodetic measurement campaigns (eg, CONT94) and the contribution from ocean tides as predicted by tide models derived from satellite altimetry observations (eg, TOPEX / Poseidon) in the same period, we got the lump-sum values ​​of PM and LOD.The computed total effects and the observed PM and LOD are generally agree agree with each other .In another word , the difference of the observed PM and LOD and the model only considering ocean tides, at all tidal frequencies (long periods, diurnals, and semidiurnals) contains the contributions of the lateral density heterogeneity of the mantle. Study of the effect of mantle density heterogeneity effect on torque-free Earth rotation may provide useful constraints to construct the reference earth model (REM), which is the next major objective in global geophysics research beyond PREM.