Three Ni-Mo/Al2O3 catalysts were used in this work to study the hydrodemetallization reactions on Maya heavy crude oil. Two reactions, HDM and asphaltenes conversion (HDAs), were studied at four different temperatures (653, 673, 693 and 713 K) and four space velocities (0.5, 1.0, 1.5, and 2.0h(-1)) at constant pressure and hydrogen-to-oil ratio of 6.86 MPa and 890.47 ml/ml, respectively. Removals of sulfur and nitrogen were also studied at the same conditions. Activities were related to operating conditions and catalyst properties. The activity of the catalysts was different at high and low temperatures. The optimum pore diameter for removal of sulfur and nitrogen compounds was smaller than that required for metals and asphaltene conversions. It was observed that catalyst having high pore volume in the 100-250 Angstrom region showed high HDM activities. Apparent activation energies were also determined for each reaction and each catalyst.