Rare earth (RE) ions-doped mayenite Ca12Al14−xRExO33 nanopowders (where RE = La and Gd and x = 0–1.0) were synthesized using the oxalate precursor technique. The as-prepared precursors were calcined at 800 °C for 2 h. Obviously, all RE-doped Ca12Al14−xRExO33 possessed a well-crystalline cubic mayenite phase till RE content of 0.8. The crystallo-chemical aspects including crystallite size, lattice parameters, theoretical X-ray density and bulk density were robustly on RE nature and ratio. The microstructure and the average grain size were significantly influenced by the RE kind and content. The high transparency of Ca12Al14−xRExO33 over 80% was found to be evinced in the visible wavelength range of 400–800 nm. Besides, the incorporation of RE cation minimized the direct band gap energy from 4.42 eV for pure mayenite to 3.85 and 3.59 eV with x value 1.0 of La3+ and Gd3+ ions. The photoluminescence spectra of pure mayenite nanoparticles showed that the band edge emission (λexc = 248 nm) with an intense visible emission band at 360 nm was detected. Otherwise, the band edge emission showed a slight shift toward short wavelength due to the substitution Al3+ by RE3+ ions. Such results open a new avenue for application of mayenite as a good candidate for transparent low-temperature electron conductor for optoelectronics applications.