The present paper deals with the reaction of the potassium titanate fiber on four kinds of pure metals (aluminum, nickel, titanium and copper) in air and vacuum at elevated temperatures, in order to find the applicability of the fiber to MMC. Results obtained are summarized as follows: (1) The fiber does not react on the aluminum matrix up to the melting point of the matrix. Beyond the point, however, it is found by EPMA line analysis that potassium in the fiber disappears. (2) Up to 1123 K, there is no detectable reaction between the fiber and the nickel matrix. (3) The fiber dose not react in vacuum on both titanium and copper matrices below the melting point of them, while a violent reaction between the fiber and the matrices occurs in the atmosphere at temperatures higher than 1023 K. The above mentioned results suggest that the potassium titanate fiber can be applied as a reinforcement in the aluminum and nickel matrices when the MMC is fabricated in air, as well as in the titanium and copper matrices when fabricated in vacuum.