CoSb3 compounds were prepared by the arc melting and their thermoelectric properties were investigated at 300K-600K. Annealing effects were examined and they were correlated to phase transformation and homogenization. Undoped CoSb3 showed p-type conduction and intrinsic semiconducting behaviour at all temperatures examined. Thermoelectric properties were changed with constituent phases because gamma-CoSb2, beta-CoSb and Sb are metallic or semimetallic phases while delta-CoSb3 is semiconducting phase. Thermoelectric properties were remarkably improved by annealing in vacuum and they were closely related to phase transitions. Single phase delta-CoSb3 was successfully obtained by annealing at 400degreesC for 24hrs.