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Nanopowders of up-conversion SrGd 2 O 4 orthorhombic (Pnma) phase co-doped with different Yb3+ (1, 2.5 and 5 at%) and constant Er3+ (0.5 at%) ions were successfully prepared via sol-gel assisted combustion. Rietveld refinement indicated unit cell lattice parameters increase with Yb3+ and Er3+ ions doping. Scanning transmission electron microscopy with corresponding energy-dispersive X-ray spectroscopy revealed that obtained powders are composed of agglomerated nanoparticles that have a uniform distribution of all constituting elements. Photoluminescence measurements implied intensification of the up-conversion (UC) emission in the visible part of spectrum with the increase of Yb3+ content, which is followed by a significant change in the green to red ratio. Two-photon UC processes are established as a result of Er3+ f-f electronic transitions: green emission at 523 and 551 nm (2H 11/2 , 4S 3/2 → 4I 15/2) as well as a red emission at 661 nm (4F 9/2 → 4I 15/2). The highest value of absolute quantum yield of 0.055% is determined for SrGd 2 O 4 nanoparticles doped with 0.5 at% of Er3+ and co-doped with 5 at% of Yb3+ (λ exc = 976 nm, power density 200 W/cm2). • Up-conversion SrGd 2 O 4 :Yb, Er nanoparticles are prepared via sol-gel combustion. • Random distribution of Yb over two Gd sites in Pnma structure is determined. • Two photon up-conversion (green and red) is affected by the Yb/Er ratio. • Quantum yield of 0.055 % is measured for radiation densities over 200 W/cm2. [ABSTRACT FROM AUTHOR]