The uniaxial hot compression tests of casting Mg-2.5Sn (wt.%) and Mg-2.5Sn-0.2Y (wt.%) alloys were conducted on an MTS-CMT5I05 universal testing machine at temperatures ranging between 250~400℃ and the strain rate of 10_-4~10_-1 s_-1 with strain of 40%. The effect of yttrium (Y) addition content (0.2 wt.%) on the hot compression behavior of the casting Mg-Sn alloys were investigated by true stress-strain curves, peak stress, microstructures, constitutive equation, and DMM processing maps. Results showed that the Y addition increased the peak stress of 30% at low strain rate, but made any sense at high strain rate. Deformation mechanism was changed from the lattice diffusion to dislocation slip and climb. In addition, Y retarded the dynamic recrystallization, reduced the efficiency of power dissipation and expanded the flow instability area.