CoSb3 powders were synthesized by mechanical alloying (MA) with Co and Sb powders as starting materials. The influence of milling speed and duration on the solid reaction were systematically studied. The XRD results indicated that the CoSb3 phase increased with prolonged MA-time, but extensive MA resulted in the appearance of CoSb2 and the decomposition of CoSb3 to CoSb2 and amorphous Sb at a constant MA-speed. Increasing the MA-speed can shorten the incubation time of the reaction, but doesn't influence the MA solid reactions themselves. For MA processes that give a specific state of alloying, the MA-speed and the MA-time should satisfy the relationship: ω3.8t=C(constant), which expresses the relationship of equivalency between MA-speed and MA-time, indicating that the supplied energy is being accumulated during MA. The MA mechanism concerning the formation of CoSb3 has been discussed on the basis of MA from a viewpoint of non-equilbrium thermodynamics.