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Development of semiconductor laser diodes (LDs) is interested due to their use as transmitters to generate digitalized signals, 0 and 1, by turning on and off their output power in optical communication systems. Initially, multimode lasers such as Fabry-Perot (FP) LDs were used as transmitters whereas chromatic dispersion in optical fibers limited the transmission speed and distance while using this broadband or multimode source. Thus, semiconductor lasers with single mode operation is essential for the long-haul transmission at high bit rates and wavelength division multiplexing (WDM) where Distributed Feed Back (DFB) lasers are widely used as a light source having a single longitudinal mode (SLM). Recently, high single mode yield in uncooled DFB laser diode fabrication is required for production of low-cost fiber optic communication components [1]. However, it is well known that complex (gain, loss)-coupled DFB LDs have much higher single mode yield than the yield of pure index-coupled ones for the cost of complexities in device structures and fabrication processes [2], [3]. In this work, complex-coupled DFB laser under loss coupling mechanism with multi-quantum well (MQW) structure and absorptive region inserted in the Bragg index grating was fabricated in a simpler process and the effect of quantum well numbers on the optical characteristics (threshold current, quantum efficiency) of laser diode was measured at room and high temperatures for the evaluation. Fig. 1(a) shows the schematic view of the DFB laser diode. The device was grown in a MOCVD process at 600 o C and 60Torr with vertical type reactor of high speed rotating susceptor using sources such as group V (AsH 3 : 20%, PH 3 : 100%), group III (TEGa, TMIn), and dopants (n: SiH 4 , p: DEZn); InP and InGaAsP of 1.1µm and 1.25µm, MQW (1% compressive strained well) and Pd diffused hydrogen as carrier gas. The InP grating was formed by chemical etching using a holographyically aligned mask with typical depth of 60~70nm. To investigate the reliability and stability of fabrication variables including growth quality of MQW, regrowth quality for a current blocking layer, and other same fabrication processes in the development of DFB LD, FP LD was used which shows exactly the same fabrication processes except the Bragg index grating below active region. Detailed fabrication processes are described in [4]. Fig. 1(b) is the result of double crystal x-ray diffraction (DCD) spectrum for 1% compressive strain MQW layer. To examine the effect of well numbers without the noises from grating region and the following over growth issues, we first fabricated a multi-quantum well multimode FP LD for comparison before a single mode DFB LD. In Fig. 2, as the number of quantum well decreased from seven to six, there is a decrease of threshold currents and increase of quantum slope efficiencies both at 25°C and 85°C for FP laser. Afterwards as shown in Fig. 3, the effect of quantum well numbers on threshold current and slope efficiency is measured at two different temperatures...