The effects of rapid thermal annealing on the electrical and structural properties of Ru/n-InP Schottky diode have been investigated by current–voltage (I–V), capacitance–voltage (C–V), X-ray diffraction (XRD) and secondary ion-mass spectroscopy (SIMS) techniques. Results showed that the Schottky barrier height of the as-deposited Ru/n-InP has been found to be 0.53 eV (I–V) and 0.69 eV (C–V). The Schottky barrier height increased to 0.54 eV (I–V) 0.73 eV (C–V), 0.56 eV (I–V) and 0.78 eV (C–V) after annealing at 300 and 400 °C for 1 min in nitrogen ambient. Further increase in the annealing temperature up to 500 °C resulted in the increase of barrier height to 0.57 eV (I–V) and 0.80 eV (C–V). However, after annealing at temperature 600 °C, the barrier height decreases to 0.51 eV (I–V) and 0.67 eV (C–V). Based on the results of XRD and SIMS studies, the formation of indium phases at Ru/n-InP interface could be the reason for the increase of barrier height upon annealing at 500 °C. The atomic force microscopy results showed that the surface morphology of the contact annealed at 600 °C is fairly smooth with a root mean square roughness of 1.8 nm.