Abstract: The effect of a Mg promoter on the physical and chemical properties, as well as catalytic performance of the Co/Mg/HZSM-5 catalyst for the partial oxidation of methane(POM) to syngas was studied by XRD, H2-TPR, TEM, Raman, XPS and activity measurements. The activity and stability of the Co/HZSM-5 catalyst was effectively improved by Mg modification. At T=750 ℃ and SV (space velocity)=1.0×10⁵ mL·h^-1·g^-1 the Mg-modified catalyst exhibited high activity and od stability during a long run. The unmodified catalyst rapidly deactivated after 10 h on stream. Catalyst deactivation was mainly due to the transformation of Co0 into CoAl2O4 as indicated by TPR and XPS. For the Co/Mg/HZSM-5 catalyst the Co species, in addition to ones existing as Co3O4, reacted with the Mg promoter to produce MgCo2O4. This structure, after reduction, led to a higher dispersion of Co metal, compared with the Mg-free catalyst. From the results of the characterization and activity measurements, the relationship between catalyst structure and performance was discussed.