Potassium hexatitanate fibers having a tunnel structure and a free potassium content of 5 ppm or less can be produced by mixing together a titanium containing compound and a potassium containing compound in a ratio represented by the formula K.sub.2 O.nTiO.sub.2 (wherein n=from 2 to 4); firing the mixture at 900° to 1,200° C. to produce mass of potassium titanate fibers; dipping the mass of product in either cold or hot water to disintegrate the mass of potassium titanate fibers into individual single fibers; adding an acid to the slurry to adjust the pH value to 9.3-9.7, thereby changing the composition of the potassium titanate fibers so that the molar ratio of TiO.sub.2 /K.sub.2 O is in the range of from 5.95 to 6.00; heating the fibers at 950° to 1,150. degree. C. for 1 hour or more; and washing the fibers with an acid. The potassium hexatitanate fibers, which have a minimal free potassium content, i.e., 5 ppm or less, can be suitably used as a reinforcing material for polyester thermoplastic resins, polyphenylene sulfide resins, liquid crystal polymers, aluminum alloys, magnesium alloys and so forth, all of which are easily affected by free potassium.
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