PURPOSE: To obtain a reinforcing fiber capable of providing a polymer composite material having high oxidation stability by coating a fiber with a mixed solution of a specific metal alcoholate and a basic metal carboxylate and carrying out a heat treatment to form an oxide layer on the surface of the fiber.

CONSTITUTION: A metal alcoholate the formula Mx(OR)y (M is a metal having a valence of 2 to 4; R is an organic radical; x is 1; y is 2-4) is mixed with a proper amount of a basic metal carboxylate of the formula [R' is an organic radical; M' is a metal having a valence of 3 or 4; X is BO3-, SO4--, Cl-, NO3- or R"CO2- (R" is an organic radical; n and m correspond to valences of (M'-1) and X, respectively; Z has a value sufficient so that the R'-CO2M'(OH)n is water- soluble) to prepare a solution in which the metal alcoholate is solubilized. The solution is applied to a fiber and is exposed to a temperature sufficient to form an oxide surface. Ni, Fe, Ti, Al, Zr and Si, especially Al is preferable as the metal M.