Abstract: A potassium hexatitanate whisker having a tunnel structure containing aluminum and niobium impurities in such amounts that Al.sub.2 O.sub.3 /Nb.sub.2 O.sub.5 molar ratio is 0.6 or higher and composite materials using the same. Also disclosed are processes for producing composite materials using the potassium hexatitanate whiskers having a tunnel structure with a light alloy or a thermoplastic resin.