An efficient and straightforward approach is proposed to produce extreme ultraviolet frequency combs by using high-order harmonic generation from diatomic molecules driven by surface-enhanced high-repetitionrate femtosecond pulses. For a nonpolar molecule, the maximum ͑minimum͒ energy conversion efficiency is observed as the molecule is oriented at an angle of 27°͑108°͒ relative to the surface. For a polar molecule, the frequency components of the generated ultraviolet frequency combs are

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... d as both odd-and even-order harmonics are generated, which shows different orientation dependence and shifts from the nonpolar molecule. The generated ultraviolet frequency combs are mainly emitted with angles smaller than 30°close to the surface, while there is a noticeable emission probability around the directions perpendicular to the surface when the molecules are oriented with large angles.