At high temperature, biomass steam gasification generates tars and more particularly Polycyclic Aromatic Hydrocarbons (PAHs). To assess the ability of olivine-based catalyst to eliminate the tars by steam reforming, alpha-methylnaphthalene (MNP) was chosen as model compound. The Ni/olivine has shown a good ability to reform the MNP, much better than olivine alone. Moreover, catalyst water pre-treatment has proven its efficiency. Thereby, this investigation has emphasized secondary reactions such as water gas shift reaction, and an overall mechanism has been proposed, taking into consideration the important role of adsorbed oxygen species coming from water catalytic dissociation over nickel metal. In addition, catalyst activity and stability have been discussed on the basis of X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with EDX analysis, carried out on the samples before and after experiments. On the prepared fresh catalysts, we have shown the presence of NiO-MgO solid solution, formed on the surface of olivine support. Then, after the experiment, both Ni metal (Ni-0) and Ni-Fe alloys have been observed.