Abstract: The influences of CaO on the formation and growth of iron whiskers and their mechanism in the reduction of iron oxide under CO atmosphere were studied.In situ observations on the development of iron whiskers were carried out using a stereo optical microscope and a hot stage under CO atmosphere.The reduced samples were analyzed by scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS) and Fourier transform infrared spectroscopy(FTIR).It is indicated that the formation and growth of iron whiskers proceed in the stage of FeO being reduced to Fe and CaO has a negative effect on the reduction of iron oxide.When the mass fraction of CaO mixing into the sample is more than 8%,iron whiskers can not be observed.Therefore,controlling the amount of CaO mixing into the sample can inhibit the formation and growth of iron whiskers and reduce ore grain agglomeration.