Our newly constructed ultrahigh vacuum atomic force microscope (UHV AFM) with a sample cleaving mechanism is described. The cantilever is scanned by a tube scanner and its deflection is detected by the fiber-optic interferometer method. Our AFM has dual-axes piezoelectric micropositioners. The x-axis micropositioner is used for sample cleavage, which forces the sample to press a block for cleavage. The z-axis micropositioner is used for coarse positioning of the cantilever to the sample. All the components of the fiber-optic interferometer, except for the optical fiber, are located outside the chamber, so that a very compact and simple UHV AFM design is possible. The performance of the instrument is discussed with images of both graphite and GaAs (110) surface obtained in UHV.