In this work, a marine microalga Tetraselmis suecica was pyrolyzed in a fixed-bed reactor in the presence of a series of zeolite (Si/Al = 30) supported Ni catalysts to increase the hydrocarbon composition of the pyrolysis oil by a two-step process - devolatilization followed by catalytic treatment. A 3 wt% Ni-loading on zeolite showed the highest hydrocarbon content of 55.38%, in which aliphatic hydrocarbons and aromatic hydrocarbons were 35.21% and 20.17%, respectively. Besides, the oxygen-containing compounds decreased from 42.88% to 9.55% while the nitrogen-containing compounds decreased from 40.68% to 35.07%. The activity of the Ni-loaded catalysts was found to increase up to 3 wt % Ni-loading and decrease at higher loadings in similar lines to catalyst acidity measurements. Besides, the catalytic activity decreased with an increase in crystallite size of Ni at higher Ni loadings. The results indicate that the Ni-loaded catalysts can deoxygenate pyrolysis oils, but it is necessary to address the removal of the nitrogenous compounds. Pyrolysis temperature and Ni-loading play an important role in the removal of oxygen and nitrogen from the bio-oil. The quality of the bio-oil can be affected by the presence of strong acid sites, nickel-loading and the pore size. (C) 2019 Elsevier Ltd. All rights reserved.