This paper presents the deformation characteristics of honing head in order to improve the accuracy in honed hole diameter of in-process automatic sizing in honing. Thermal deformation by the warmed honing oil or by the heat generated during machining is experimentally examined. Elastic deformation by honing force is numerically and experimentally analysed. The results indicate that the thermal deformation of honing head shows nearly iso-thermal expansion. The elastic deformation of honing head is precisely obtained by F. E. M. calculation. Torque component of honing force is the main factor which has a strong influence on the elastic deformation. Thrust component of honing force has very little influence on the deformation.