Domain structure of several poled (K0.50Na0.50)(1-x)LixNbO3 ceramics (with chemical compositions of x=0.03, 0.065, and 0.08, respectively) was investigated by means of observing the domain patterns with an acid-etching technique. Among the three ceramics, the one with x=0.03 is of orthorhombic phase and the other two are of tetragonal phase at room temperature. It was found that these ceramics possess distinctly different features of domain patterns and show a large difference in the time-aging stability of piezoelectric properties. For the ceramic with x=0.03, domain patterns consist of simply one single set or a few sets of parallel stripes inside the polycrystalline grains. In contrast, for those with x=0.065 or 0.08, herringbone-type patterns and a large number of watermarks are additionally observed. Furthermore, the ceramic with x=0.03 was confirmed to have a much better time-aging stability of piezoelectric properties than the other two. The results indicate that domain structure is more stable in orthorhombic phase than in tetragonal phase.