Abstract: According to established theories about the origin of oil,temperature d geothermal gradient are among the important conditions for the transformation of organic matter into oil.In this paper,the author has made a correlation between the location of large oil fields of the world and the global distribution of heat flow values which shows that the heat flow values of oil fields of the world range approximately 45mwm^-2-75mwm^-2.The temperature at the depth where oil is formed has also been calculated.It is interesting to note that the values of temperature obtained agree well with the data from 32 oil fields in China.In addition,the author has worked out a rough estimate of heat flow values and geothermal gradients required for generation of petroleum for the different ages in the geologic past for comparative purpose.It may be noted that the depth at which oil is formed,the rate of transformation of primary source material into petroleum and the changes in the property and composition of oil are all related to the increase of geothermal gradients caused by the increase of terrestrial heat flow values.In other words,a large oil pool requires both good geological conditions and favourable terrestrial heat flow values.The author believes that we may make use of the latter as a guide to the discovery of new pools or extension of the old,as well as to the location of prospective areas.It follows therefore that investigation of terrestrial heat flow values is a new field of study in petroleum exploration.