Three contradictive conclusions on the effect of pore pressures on organic matter's maturation(OMM) have been obtained from previous experimental studies: pore pressures may play a positive, null and negative role on organic matter's maturation. After systematic review of the literature about this subject, the authors found that these contradictive conclusions  probably resulted from different experiment conditions, especially the equipments' structures and “pressure” loaded on samples. After an analysis of  the mechanisms of the effects of pressures on OMM, it is denoted that when the increase of “pressure” on the samples is the increase of effective stress, the stress acts directly on solid OM and  the maturating rate will be accelerated. On the Contrary, if increasing “pressure” acts on pore fluid to augment pore pressure, then the maturating rate will be decelerated. It is previously proved that the vitrinite reflectivity corresponding to the OM appears relatively lower in these experiments of both types of cases. In actual basins, the effect of overpressures on OMM must be so different that the overpressuring occurs underground with different mechanisms at different burial depths and sedimentary environments.