This work deals with the effect of catalyst preparation method of the mixed Co, Mg and La oxide catalysts on their structure and catalytic properties for ammonia decomposition. Two methods are used for catalysts preparations impregnation and co-precipitation (in air and in pure 02 atmosphere), The Mg/La = 2 molar ratio and 5 wt% of cobalt content was maintained same in all catalysts. The catalyst performance was evaluated in the temperature range 300-550 degrees C at atmospheric pressure. The prepared catalysts were characterized by BET, XRD, TPR, XPS, CO2-TPD and SEM techniques. No pronounced differences were observed in BET among the catalysts. It was found that the 5CML-OXY (5 wt%Co over Mg La catalyst prepared by co-precipitation method in oxygen atmosphere) has superior activity among the other catalysts. This could be attributed to availability of easily reducible cobalt species determined by TPR studies and enhanced interaction between Mg and La determined by SEM and XPS. The moderate basic site density determined by CO2-TPD results was also increased in 5CML-OXY catalysts compared with other catalysts. These consequences are might be one of the reasons for enhanced activity of 5CML-OXY catalyst compared to other catalysts. Hence catalyst preparation by co-precipitation in oxygen atmosphere is the best method which might be one of the parameters that influenced on catalytic properties of the cobalt on MgO-La2O3 system, for ammonia decomposition. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.