Our previous studies have shown evident technical superiorities of oil shale pyrolysis in a newly devised fixed bed reactor with particularly designed internals (Fuel Processing Technology, 138, 2015: 147-155). This study is furthered to optimize the operating conditions by focusing on the reaction pressure to enhance the yield and quality of shale oil in the fixed bed reactor with internals. Reducing the pyrolysis pressure increased the shale oil yield in comparison with the pyrolysis under atmospheric pressure. This influence due to reducing reaction pressure appeared to be higher in the reactor with internals than in the reactor without any internals. Moreover, there were higher yield of aliphatic components and fewer aromatic species in the reactor with internals, essentially indicating the suppressed secondary reactions to the produced primary shale oil in the new reactor. In this reactor the shale oil yield reached 97.57% of the Fisher Assay yield (8.24 wt.% dry basis) at a furnace heating temperature of 1000 degrees C under reduced pressure conditions. Consequently, there is actually some synergistic effect between internals and reducing pressure so that adopting internals and using vacuum condition can grant the indirectly heated fixed bed reactor great potential to achieve high yield and high quality of shale oil in oil shale pyrolysis. (C) 2016 Elsevier B.V. All rights reserved.