The influence of the volume fraction of long-period stacking ordered structure (LPSO) on the microstructure and mechanical properties in three extruded Mg100-3x Y-2x Zn (x) alloys (x = 0.5, 1 and 1.5 at.%) has been studied. Two structures of LPSO phase coexist in these extruded alloys, 18R and 14H. The 18R structure transforms to 14H structure gradually in the course of the extrusion process. For the three alloys, the grain size in the vicinity of LPSO phase particles is refined because of a particle-stimulated nucleation (PSN) mechanism. The reinforcing effect of the LPSO phase is active up to 523 K. Above this temperature, grain size effect becomes important. Accordingly, MgY1Zn0,5 extruded alloy shows the Highest mechanical strength for temperatures greater than 523 K.