We have shown that piezoresponse force microscope (PFM) is an effective tool for observing 3-dimensional domain structure formed in ferroelectric crystals. The PFM is based on the measurement of the surface displacement induced by inverse piezoelectric effect by using conductive cantilevers. The small tip of the cantilever plays an important role not only in applying electric field as a top electrode but also in detecting piezoelectric displacements just beneath the tip. In the vertical mode, the component of spontaneous polarization (P_s) parallel to the electric field can be mapped, while the in-plane mode provide an image of the component of P_s parallel to the crystal surface. In a PbTiO₃ crystal, 90^o domain as well as 180^o domain with no interface charge was observed. PFM observations of a layered Bi₄Ti₃O₁₂ suggest that charged 180^o domain walls with tail-to-tail and head-to-head configurations strongly interact with oxygen vacancies, which leads to a smaller remanent polarization.