In the present work, reduced presulfided NiMo/gamma-Al2O3, the conventional hydrotreating catalyst, was evaluated for green diesel production via hydrodeoxygenation of unrefined microalgae oil in a microreactor, mimicking the single channel of a monolithic reactor. The effect of reactor inner diameter on space-time yield of hydrocarbon and microalgae oil conversion was studied first to confirm the superiority of the microreactor for three-phase reactions. Based on the external and internal mass transfer limitation analyses, a range of process conditions without mass transfer limitation was determined for catalyst evaluation. The results showed that NiMo/gamma-Al2O3 is deactivated due to the accumulation of produced oxygenated intermediates in hydrodeoxygenation reaction, and its selectivity to even-numbered carbon hydrocarbon produced from hydrodehydration correlates with the catalyst activity. The catalyst activity and life can be preserved by increasing hydrogen to oil ratio, residence time, reaction temperature, and pressure, which will decrease the adsorption of oxygenates on the catalyst surface. For the reaction condition: 500 psig H-2, 360 degrees C, H-2/oil ratio of 1000 SmL/mL, and residence time of 1 s, the initial catalyst activity was maintained without any signs of deactivation for at least 7 h and the obtained C13 to C20 hydrocarbon yield was 56.2%, with a carbon yield of 62.7%, nearly complete conversion (98.7%) of microalgae oil, and HC(2n)/HC(2n 1) ratio of 6.