Lanthanum (La3+) ion doped zinc ferrite nanoparticles were synthesized by combustion method using glycine as fuel. The as-synthesized ZnFe1.96La0.04O4 nanoparticles were subjected to annealing temperature of 900 oC for time intervals of 2 h, 6 h, 12 h, 24 h and 48 h. The as-synthesized and annealed ZnFe1.96La0.04O4 nanoparticles were characterized as a single phase with normal spinel structure. The surface morphology of these nanoparticles were found to be non–uniform and agglomerated with fine pores/voids. The induced strain and dislocation density were reduced with increasing annealing time which enhanced crystallinity and increased grain size. The values of optical band gap calculated from UV/Vis/NIR spectra of the as-synthesized and annealed ZnFe1.96La0.04O4 nanoparticles were found to decrease with increasing annealing time. They range from 2.48 to 2.19 eV from the simple method and 2.42–2.12 eV for direct and 1.87–1.71 eV for indirect from Kubelka–Munk function method. The optical band gap in ZnFe1.96La0.04O4 nanoparticles can be tuned as function of varying annealing time and it seems to correlate with induced strains in the nano-crystallites. Therefore, the tuning of optical band gap with just changing the heating treatment of ZnFe1.96La0.04O4 nanoparticles may make them suitable photocatalysis.