Abstract: The current research efforts are focused on development of practical mid-IR laser devices that can be used in high resolution spectrometers for sensitive detection of specific gas molecules. Improvements in processing and thermal management are expected to yield room temperature lasing in the near future. In this research, the effects of laser cavity length on a critical thermal operational parameter, the characteristic Temperature (T₀) was studied was studied for this PbSe/Pb_0.934 Sr_0.066 Se mid-IR quantum well structure at three temperature ranges 77<T<150 K, 150<T<300 K and 77<T<300 K. It was noticed that T₀ is high for short cavity lengths then decreases to an almost constant value after some critical length of 100 μm. The effects of quantum efficiency were noticeable for short cavity lasers with a reduction of 55-62% in T₀ for the 77 K≤T≤150 K and 150 K≤T≤300 K and 68-73% for the 150 K≤T≤300 K range. However, as the cavity lengths increased above the critical value the effects are almost constant. The data were best fitted to a second degree polynomials which can be used to determine thecharacteristic temperature values as a function of cavity length.