In this work, the influence of shale grain size of 0.20-0.60, 0.60-1.20,1.20-3.33, and 3.33-5.60 mm were carried out on the oil shale pyrolysis in a semi-continuous fluidised bed and 50 gm fixed bed reactor under nitrogen and nitrogen/steam pyrolysis atmosphere. The final pyrolysis temperature was 520 degrees C. heating rate 12 degrees C min(-1), vapour residence time of 25 and 28 s for the fluidised bed and fixed bed pyrolysis, respectively, and shale residence time of 45 min. The oil yield was increased as the particle grain size was increased from 0.2-0.6 to 3.33-5.6 mm under both nitrogen and nitrogen/steam pyrolysis atmospheres. Increasing the particle grain size caused a decrease in the derived hydrocarbon gases and an increase in the carbon and hydrogen content in the derived oil. Decreasing the particle grain size (in the studied range) caused an increase in the PAH concentration in the derived oil. This increase has been suggested as a result of the aromatisation of the derived oils by the secondary decomposition reactions. (C) 2008 Elsevier Ltd. All rights reserved.