Nanoscale structures and physical properties are of fundamental interest, and thus variations in scanning probe microscopy techniques have been developed to understand nanoscale physical phenomena in basic research and potential technological applications. 1) Very recently, scanning near-field ultrasound holography has been developed that provides nanoscale-resolution images of the internal substructures of microelectronic materials and malaria parasites in red blood cells. 2) In the field of ferroelectrics, nanoscale domain structures of ferroelectric thin films, ceramics and single crystals can be imaged and controlled directly using piezoresponse force microscopy. 3–10) Besides, acoustic mode scanning force microscopy including atomic force acoustic microscopy, 11,12) ultrasonic force microscopy, 13–16) and scanning probe acoustic microscopy 17–21) have the advantage of visualizing nanoscale domain structures, elastic properties or subsurface configurations. In the present study, a type of alternating-force-modulated atomic force microscope (AFM) in the acoustic mode was developed by exciting acoustic waves on the piezoelectric transducer, which is fixed on a cantilever in the commercial AFM. This acoustic mode AFM was used to visualize the ferroelectric domains in BaTiO3 single crystals. An alternating force-modulated AFM, functioning in the acoustic mode, was set up by modifying a commercial AFM (SPA 400, SPI3800N, Seiko, Japan). A schematic sketch of the modified AFM is shown in Fig. 1. An external sinusoidal signal provided by a function generator (33120A, Agilent) was applied to the piezoelectric transducer attached to the cantilever, thereby inducing a periodical vibration in the cantilever/tip system. This modulated-tip vibration driven by the function generator emits a local stress field underneath the tip, resulting in an acoustic vibration inside the sample, which gives rise to a remarkable acoustic response of the ferroelectric microstructure due to physical phenomenal effects. The resultant acoustic signal carrying ferroelectric information was converted into an electrical signal through a piezoelectric lead zirconium titanate (PZT) ceramic transducer bonded onto the sample, and then fed to a lock-in amplifier (Model 7280 DSP, Signal Recovery Instrumentation) for detection. For the experiment, a 2-mmthick 90-mm-long Ti/Pt-coated silicon cantilever (Micro Mash, NSC12-B) with a spring constant of 14 N/m and a resonance frequency of 315 kHz was used. For the present study, we chose a 0.3-mm-thick (001)-oriented BaTiO3 single crystal. Figures 2(a) and 2(b) are a topography image and the corresponding acoustic image of a BaTiO3 single crystal, respectively, at a modulation frequency of 133.42 kHz. It can be clearly seen that the acoustic features are nearly the same as those of the topography image, indicating only slight variations in the surface microstructure after sample polishing. Figure 2(c) shows an acoustic image taken at a frequency of 1.419 MHz in the same scanning area. Unlike Fig. 2(b), a different acoustic response is clearly seen in Fig. 2(c), exhibiting remarkable acoustic contrasts in parallel strips about 300 nm in width, which are obviously the 90 ferroelectric domains in the BaTiO3 single crystal. The striped patterns are more distinguished in another large scanning area of 10 10 mm 2 measured at a frequency of 1.419 kHz, as shown in Fig. 3. The intercrossing domain phenomena shown in Fig. 3(b) reflect the domain state distribution at different depths throughout the specimen. Such phenomena were observed in Han and Cao’s studies of the averaged domain configurations using a polarized optical microscope, in which the domain configurations at various depths were exhibited in an optical image by adjusting the focus. 22)