In this paper, the influences of particle size, grade and pyrolysis temperature on the oil yield have been evaluated in a laboratory scale reactor. Five categories of particle sizes from two different oil shale samples were pyrolyzed by employing a fixed bed retorting system. The reactor and the oil shale sample were heated at a constant rate and nitrogen gas was used to purge the sample, continuously, in order to remove the pyrolysis products from the reactor as well as to reduce secondary reactions. The liquid products were condensed and collected in a series of cold glass-traps and the off-gases analysed for their hydrocarbon and non-hydrocarbon species. Subsequent experiments were carried out, employing a thermogravimetric analyser, using only the four smallest particle sizes under similar conditions as applied to the fixed bed retort. The activation energy was determined by using the integral method. The pyrolysis of the investigated shales was found to comply with first-order kinetics within the limits of experimental error. Increasing the particle size resulted in a small rise in the liquid oil yield, but simultaneously the total gaseous production was decreased.