An investigation was carried out to understand the phase evolution and study the structural, morphological, optical and electrical properties of Co-Sb alloys fabricated by two different approaches: (a) thermal annealing and (b) ion-beam mixing followed by post annealing. The as-deposited and 100 MeV Ag ion beam irradiated Co/Sb bilayer thin films were subjected to thermal annealing from 200 to 400 °C for 1 hour. The Rutherford backscattering spectrometry (RBS) results showed partial mixing for the thermally annealed films and complete mixing for the irradiated and post annealed films at 400 °C. The XRD and RAMAN measurements indicated the formation of Co-Sb alloy, with ∼70% concentration of CoSb3 phase in the irradiated post annealed sample at 400 °C. The band gaps of the annealed and post irradiated annealed Co-Sb alloys were determined using UV-visible spectroscopy. Electrical and thermoelectric power measurements were performed in the temperature range of 300-420 K. It was observed that the alloys formed by ion-beam induced mixing exhibited higher electrical conductivity and thermoelectric power than the as-deposited and thermally annealed Co/Sb bilayer thin films.