In the present article, we investigated theoretically the total absorption coefficient related to inter conduction subband transitions in symmetric In.1Ga.9N/GaN double rectangular quantum wells within the effective-mass and parabolic band approximations making an allowance for temperature and structure's size influences. The finite difference method is adopted to solve numerically one-dimensional Schrödinger equation and to obtain the six first low-lying allowed eigen-values and their corresponding eigen-functions in view of finite potential barrier height, effective-mass and dielectric constant mismatches. The optical absorption coefficients are computed based on the compact density matrix approach within an iterative procedure. The numerical results are reported for two different values of temperature. Obvious temperature impact is obtained mainly for narrow coupling barrier compared to large one. Apart from 3S–3P transition and regardless the coupling width, momentous red-shift is found with increasing temperature. Moreover, 2S–2P, 2S–3P and 3S–3P associated TOAC spectra are blue-shifted while all others show a substantial red-shift with increasing the coupling width. Also, it is found that resonant optical absorption characteristics can be tailored by an appropriate selection of temperature and structure's size leading to new prospective for good performance infrared optical devices, optical modulators, solar cells and laser applications.