Within Ginzburg-Landau theory we calculate the surface tension of the superconducting/normal interface of a type-I superconductor. For the reduced surface tension Gamma(SC,N) we derive the low-kappa expansion Gamma(SC,N) = 2 root 2/3-1.02817 root kappa-0.13307 kappa root kappa+O(kappa(2) root kappa), where kappa is the Ginzburg-Landau parameter. Tne coefficient of root kappa agrees with an earlier calculation [T. M. Mishonov, J. Phys. (France) 51, 447 (1990)], but disagrees with a more recent estimate [J. C. Osborn and A. T. Dorsey, Phys. Rev. B 50, 15 961 (1994)], The coefficient of kappa root kappa differs only slightly from Mishonov's guess based on simple interpolation. We show that the expansion truncated at order kappa root kappa is already so accurate in the entire type-I regime 0 less than or equal to kappa less than or equal to 1/root 2 that derivation of higher-order terms is unnecessary.