The three-dimensional shape of secondary recrystallized grains grown in doped tungsten wires is constructed by an image analyzer. The grain boundary area per unit volume of a specimen S/V is then obtained to clarify the morphological effect of grains on the minimum creep rate during diffusional creep without grain-boundary cavitation. While the creep rate decreases as the grain aspect ratio (GAR) increases in the range of GAR<30, it increases as GAR increases in the range of GAR>30. The Raj and Ashby's model gives a satisfactory explanation to the former although it does not give the one to the latter. It is proved that the effect of the grain morphologies on the creep rate due to grain boundary sliding must be taken into account to explain the latter behavior.