This paper reports on the thermal behavior of open type of LD packages as functions of ambient temperatures, base materials, cooling conditions, and gravity directions. The results of thermal measurement and modeling were compared and analyzed to present the optimization of package design. Thermal transient methods were used to analyze the thermal behavior of laser diodes. Furthermore, finite volume methods were employed for the thermal modeling. LD samples were packaged with different base materials (Cu or Fe). Then the thermal resistance was measured with different ambient temperatures under the forced cooling condition. Also, thermal resistances at various input currents with different cooling systems were investigated. The measured thermal resistances were increased from 21.7 K/W to 27.8 K/W with the ambient temperature increased from 0°C to 50°C. The thermal conductivity of package material decreased with ambient temperature. Therefore, the poor thermal conductivity of the package material results in higher thermal resistance. The measured thermal resistances of LD packages with Cu(401 W/mK) and Fe(80 W/mK) as base materials were 15.6 K/W and 21.4 K/W, respectively. In the thermal modeling, employment of Cu as a base material was found to be more advantageous than the Fe in the thermal behavior of LD packages. There exists only a slight change of thermal resistance as functions of input currents under the forced cooling condition. But significant changes of thermal resistance with different input currents were observed under the natural cooling condition due to the sensitive change of the heat transfer coefficient with the temperature. Then, the gravity has an effect on the thermal property of LD packages.