At present,Nb3Sn superconductors are becoming more popular in high magnetic fields.The growth law of Nb3Sn phase in a planar CuSn/Nb diffusion couple has been studied,whereas the formation mechanism of Nb3Sn phase in a cylindrical CuSn/Nb diffusion couple is still controversial.The purpose of this work is to investigate the growth exponent of Nb3Sn phase at the initial stage of annealing by use of scanning electron microscopy (SEM) through which the thickness of Nb3Sn layer can be obtained.In this study,bronze-processed Nb3Sn multifilamentary wires with different annealing time were investigated.The Nb3Sn phase was formed during isothermal annealing at 670 ℃ by solid-state diffusion,which was accomplished by the movement of Sn atoms from the CuSn/Nb3Sn interface to Nb3Sn/Nb interface.However,the formation mechanism of Nb3Sn phase at the initial stage of annealing is still not well understood.Microstructural evolution of Nb3Sn phase during isothermal annealing was studied by SEM.The mean thickness of the Nb3Sn layer (△xNb3Sn) is expressed as a power function of the annealing time (t) by the equation △x2Nb3Sn =k(t/t0)n,where t0 is the unit time,k is the reaction rate constant and n is the growth exponent.The growth exponent has the average value of 0.82,which means that the formation of the Nb3Sn phase is both governed by the interface reaction and the grain boundary diffusion.