The effect of earth element Y addition on the creep properties and creep stress exponent of Mg-5.5Zn gravity cast magnesium alloy was investigated by using scanning electron microscopy(SEM),X-ray diffraction(XRD) and creep property testing at elevated temperature. The results show that, with the increase of yttrium content in Mg-5.5Zn (wt%)alloy ,three second phases,Mg7Zn3，Mg3Zn6Y(I-phase)and Mg3Zn3Y2(W-phase), are found. The variety categories and the increasing volume fraction of the second phase cause the increase of the creep resistance obviously. Compared with Mg7Zn3 phase, rare earth phases have a high melting point and it have a stronger effect for reducing the creep rate of alloy in the same given creep conditions. When the applied stresses are 50~60 MPa at the temperature 448 K, the calculated stress exponent n is 5.2, 3.2, and 2.2, respectively for the as-cast Mg-5.5Zn-Χ%Y(Χ=0.7,1.5,3.5)alloys. The results indicate that the rate controlling mechanism of Mg-5.5Zn-0.7Y alloy at 175 ℃/55 MPa and 200 ℃/55 MPa is respectively the dislocation creep and power law breakdown，and the creep mechanism of Mg-5.5Zn-(1.5,3.5)Y two alloys is dislocation viscous motion at the applied stresses of 50~60 MPa and the temperature of 448~473 K.