Variations in dielectric constant and patterns of microwave loss have been imaged in a yttrium-doped LiNbO3 crystal with periodic ferroelectric domains with the use of a scanning-tip near-field microwave microscope. Periodic profiles of dielectric constant and images of ferroelectric domain boundaries were observed at submicrometer resolution, The combination of these images showed a growth-instability-induced defect of periodic domain structure. Evidence of a lattice-edge dislocation has also been observed through a stress-induced variation in dielectric constant.