A series of LaFeO3 perovskite nanoparticles (NPs) were successfully formed by a direct sol-gel method involving different citric acid to metal ratios of x (where x = 2–16). The resulted precursors were treated via auto-combustion, followed by calcination at different temperatures (300–700 C). Size-controlled LaFeO3 NPs were getting smaller down to 18.8 nm at high citric acid ratio, showing relatively high surface area up to 27.6 m2 /g . Photocatalytic activity of the formed LaFeO3 NPs was explored for methylene blue (MB) degradation in the absence and presence of 0.5 mM H2O2 under light irradiation. Results showed that variation of citric acid to metal ratio alter the band gap and reduce the photo-generated charge carriers recombination of LaFeO3 NPs. Moreover, the LFC4-500 material (LaFeO3 synthesized using = 4, and calcined at 500 C) was the most active photocatalyst in solution at pH = 3 and 0.5 mM H2O2, with the highest efficiency > 99% within 30 min and showed excellent reusability without etching even after several cycles. Significantly, the LaFeO3 NPs characteristics were correlated with their synthesis conditions and the type of interaction with citric acid during initial stages of crystal formationA series of LaFeO3 perovskite nanoparticles (NPs) were successfully formed by a direct sol-gel method involving different citric acid to metal ratios of x (where x = 2–16). The resulted precursors were treated via auto-combustion, followed by calcination at different temperatures (300–700 C). Size-controlled LaFeO3 NPs were getting smaller down to 18.8 nm at high citric acid ratio, showing relatively high surface area up to 27.6 m2 /g . Photocatalytic activity of the formed LaFeO3 NPs was explored for methylene blue (MB) degradation in the absence and presence of 0.5 mM H2O2 under light irradiation. Results showed that variation of citric acid to metal ratio alter the band gap and reduce the photo-generated charge carriers recombination of LaFeO3 NPs. Moreover, the LFC4-500 material (LaFeO3 synthesized using = 4, and calcined at 500 C) was the most active photocatalyst in solution at pH = 3 and 0.5 mM H2O2, with the highest efficiency > 99% within 30 min and showed excellent reusability without etching even after several cycles. Significantly, the LaFeO3 NPs characteristics were correlated with their synthesis conditions and the type of interaction with citric acid during initial stages of crystal formation.