The influence of the crystallographic phase transformation on the work function was studied on a Co(0001) surface by a precise measurement of the contact potential difference. The characteristic hysteresis of the work function due to the hcp&rlarr2;fcc allotropic transformation was found. The work function changes abruptly by about 2 meV with the phase transformation while decreasing monotonically with temperature except in the transformation range. The contributions of the surface dipole barrier and the chemical potential of electrons to the work function are discussed from the point of view of cellular approximation.