We investigated thermal behavior of GaN-based laser diode (LD) packages as a function of cooling systems, die attaching materials, chip loading conditions, and optical performances. The electrical thermal transient technique was employed for the thermal measurement of junction temperature and thermal resistance of LD packages. The results demonstrate that the total thermal resistance of LD packages is controlled mainly by the packaging design rather than the chip structure itself. Significant changes in thermal resistance with input current were observed under a natural cooling system because of the sensitive change in the heat transfer coefficient with the change in temperature. Employment of PbSn as a die attachment was more advantageous over the Ag-paste in the thermal behavior of LD packages. The LD package with epi-down structure resulted in the lower thermal resistance compared to one with epi-up structure. A continuous increase in junction temperature was measured after lasing. It was attributed to an increase in the thermal resistance of LD when it took the optical power into an account. Effective input power was decreased by the lasing and led to high thermal resistance values.Index Terms-Die bonding, GaN, heat transfer coefficient, laser diode (LD) package, optical power, structure functions, thermal resistance, thermal transient measurement.