The optimum approach to determine junction temperature of GaN blue light emitting diode (LED) with spectral bandwidth is studied. The normalized spectral power distribution and junction temperature of ten GaN blue LEDs is measured at different drive current and heat sink temperature. The formulae for calculating junction temperature with Dln are obtained from the least-square fitting. Difference between the values by the established method and the forward voltage method is analyzed. The optimum n, which yields the least error, is obtained. It is shown that the spectral bandwidth Dln with different n can be used to determine junction temperature, and the error initially decreases with the decreasing n, and then increases. The optimum n depends on the accuracy of spectrometer, it is 0.2 or 0.3 when Hass2000 spectrometer is used. This research is very important to improve the measurement accuracy of GaN-type LED junction temperature.