<p>Mg-8.5Li-5Zn-1Y (wt%) and Mg-8.5Li-7.5Zn-1.5Y (wt%) alloys were prepared by vacuum melting casting method. The thermal flow behavior of the alloys was systematically investigated by isothermal hot compression tests using the Gleeble-3500 thermal mechanical physical simulation system. The tests were conducted under the conditions of the temperature of 473–593&#xa0;K and the strain rate of 0.001–1&#xa0;s⁻¹. A modified parameter polynomial constitutive model and thermal processing diagram for the two alloys were constructed. The variation pattern of the unstable region under different Zn and Y contents was analyzed and the safe processing region was determined. The influence of different areas in processing map on dislocation configuration and deformation mechanism was clarified. The results indicate that the higher contents of Zn and Y reduce the instability zone. Processing in the unstable zone only results in &lt; <i>a</i> &gt; slip, while processing in the safe zone initiates &lt; <i>c</i> + <i>a</i> &gt; slip also. The higher contents of Zn and Y reduce the critical deformation temperature for the initiation of &lt; <i>c</i> + <i>a</i> &gt; slip.</p>