Abstract A complete characterization and comparison of thermal effect between two high power fiber coupled diode laser modules operating at wavelengths of 808 nm and 975 nm. For the diode laser to operate at certain wavelength, the internal construction and the used material inside the diode must be different. Both wavelengths have a wide use in the laser design such as pumping another active medium to be lase at a new wavelength, 808 nm used to excite solid state lasers such as Nd:YVO and 975 nm used to excite fiber laser such as Yb fiber laser and Er-Yb fiber lasers, and also in applications in several fields such as military, industrial and biomedical engineering fields. For the 808 nm diode laser the output wavelength varied with changing the temperature while the 975 nm has a very large stability against the change of the operating temperature. We obtained an optical power up to 6 Watt (cw). The electrical characterization of the diode laser modules was examined by measuring the dependence of the laser driving current on the operating voltage. The optical characterization was investigated at different temperatures. These included the laser output power versus the driving current, the output optical power versus electrical input power, the laser output spectrum, the laser beam profile and the dependence of the output power on the temperature. The measured diode laser of 975 nm parameters at 25oC were (0.3A) threshold current, (35.84%) overall slope efficiency and (975.34 nm) central wavelength with line width of (1.3 nm) at FWHM. The variation of the output power, spectrum with temperature are presented. The measured diode laser of 808 nm parameters at 25oC were (1.3A) threshold current, (42%) overall slope efficiency and (807.96nm) central wavelength with line width of (3.59nm) at FWHM. The variation of the above mentioned parameters with temperature are presented