Two nonlinear optical crystal carbonates (Na4La2(CO3)(8) and CsNa5Ca5(CO3)(8) were successfully synthesized by hydrothermal method, and both of them crystallized in the same noncentrosymmetric hexagonal space group P63mc (No. 186). The structure of Na4La2(CO3)(8) consists of a three-dimensional network made up of [CO3] triangles as well as irregular [Na0.67La0.33O10] and [NaO8] polyhedra. The structure of CsNa5Ca5(CO3)(8) can be described as the standing-on-edge [CO3] groups connect the adjacent infinite [CaCO3](infinity) layers in the ab plane to construct a framework with four types of channels running parallel to [010]. The Na, Cs, and [Na0.67Ca0.33] atoms reside in these channels. The measurement of second harmonic generation (SHG) by the method adapted from Kurtz and Perry indicated that Na4La2(CO3)(8) and CsNa8Ca8(CO3)(8) were phase-matchable in the visible region and exhibited SHG responses of approximately 3 and 1 x KH2PO4 (KDP). Meanwhile, they exhibited wide transparent region with short UV cutoff edge at about 235 and 210 nm, respectively, suggesting that these crystals as NLO materials may have potential applications in the UV region.