Dysprosium (Dy) substituted nickel-ferrite (NiFe1.925Dy0.075O4) thin films were fabricated using sputter-deposition and their structural and optical properties were investigated. The grain-size (L) and lattice-expansion effects are significant on the optical properties of NiFe1.925Dy0.075O4 films. As-grown NiFe1.925Dy0.075O4 films were amorphous. Air annealing (T-a) at 450-1000 degrees C results in the formation of nanocrystalline NiFe1.925Dy0.075O4 films, which crystallize in the inverse spinel structure, with L = 5-40 nm. Lattice constant of NiFe1.925Dy0.075O4 increases compared NiFe2O4 due to Dy-substitution at the Fe-site. Band gap increases from 3.17 to 4.08 eV for NiFe1.925Dy0.075O4 films with increasing T-a from 450 to 1000 degrees C. A correlation between structure and optical properties in NiFe1.925Dy0.075O4 thin films is established. (C) 2011 Elsevier B.V. All rights reserved.