The morphologies of grain boundary grooves formed after annealing of three Mo bicrystals at the temperature close to the melting point were studied with the aid of scanning force microscopy. The ratio of grain boundary to surface energy was calculated from the surface slopes measured at the root of the groove. This ratio was about 0.3 for the grain boundary grooves with the sharp roots, and it was significantly lower than 0.3 for the grooves with the blunted roots. It was shown that the blunting of the root of the grain boundary grooves is associated with the grain boundary migration during annealing. A model of grain boundary grooving accompanied by the instantaneous boundary shift was formulated. The surface topographies predicted by the model were in a good agreement with the experimentally measured ones. It was shown that grain boundary grooving process can be completely suppressed at the singular surfaces. This, together with the grain boundary migration during annealing results in characteristic groove morphology with the blunted root and leads to a significant underestimation of the grain boundary energy calculated from surface slopes.