Abstract: The time-temperature compensation relationship as proposed in the previons paker is further proved by a time-temperature plot at the period of oil and gas maturation which is established with the time(t)and temperature(T)data at a measured reflectance R_o=0.5 from drilling cuttings in Pearl River Mouth basin.An empirical formula,logt=(144.16547/T)+0.0721409 is suggested.With this formula,the temperature T required for maturation of oil and gas generadion for various source rock formations during a certen geological time t can be estimated,and then the threshold depth H corresponding to this time t is estimated.Thus by connecting every H points corresponding to different t a curve is obtained representing oil-generating threshold depth of a certen source rock formation during the geological history.It cuts the subsidence et rve of the bottom source rock formation and the point of intersection will represent oil-generation threshold of the source formation(i.e.the beginning of oil generation).Using this method,oil-generation threshold depth of every geological period have been estimated for five suit of souce rock formations in a depression of Pearl River Mouth basin.This depth is very close to those vesults derived from many other data.Moreover,a threshold time analysis for oil generation in various geological period of soue rocks shows a relatively satisfactory result.