The scanning electron microscopic (SEM) observation on potassium bubbles, tensile and creep tests are carried out to clarify major roles of potassium bubbles in the excellent high-temperature creep resistance of doped tungsten fine wires. While bubbles, the mean diameter of which is 23 nm, are present at grain boundaries with a high area density of rho = 10(13) m(-2), a few per cent of them remain inside a grain as many arrays aligned along the wire axe. Their roles are discussed referring to the data of tensile and creep tests. The importance of controlling the grain morphology rather than strengthening the matrix is emphasized.