Scanning electron microscopic (SEM) observations on potassium bubbles and the measurement of the orientations of the wire axis for secondary recrystallized grains are made to clarify what plays a significant role in the formation of the non-sag grain structure in doped tungsten fine wires. Bubbles, the mean diameter of which is 23 nm, are present at grain boundaries with a high area density of 10¹³m^-2. It is concluded that the formation of the non-sag grain structure, which is characterized by the interlocking and the high aspect ratio of the grains, results mainly from the competitive growth of many nucleated grains through the non-uniformly distributed rows of bubbles which are aligned along the wire axis.