The temperature characteristics of the forward voltage of GaN based blue light emitting diodes are studied. We find that the temperature coefficient decreases with the increasing temperature when the temperatures is relatively high. There is an inflection point as the temperature coefficient deceases，then the temperature coefficient changes from negative to positive. If the temperature still increases at this moment，the forward voltage will increase dramatically，resulting in the failure of the light emitting diodes. This phenomenon is not obvious when the current is low. When the current increases，this phenomenon becomes more and more obvious，and the temperature of inflection point becomes lower and lower. Moreover, the forward voltage increases more quickly when the temperature is higher than the inflection point temperature. By comparing the test result of another group light emitting diodes with the same package epoxy, we find that the influence of glass transition temperature of the package epoxy can be neglected. The appearance of this phenomenon is due to the rapid increment of the equivalent series resistance，of which the main reason is the rapid deterioration of the p-type layers of the GaN based blue light emitting diode. The result shows that we can judge the quality of the GaN based blue light emitting diodes p-type layers quickly by measuring the variation of the forward voltage with temperature，which is a rapid method for the researchers and producers.