The new compounds Rb3Yb7Se12 and CsEr3Se5 have been synthesized through the reaction of elemental rare-earth metal and Se with halide flux at 680 degrees C. The structures of the compounds were determined by the single crystal X-ray diffraction method. The structures of CsEr3Se5 and Rb3Yb7Se12 are considerably similar to each other. Rb3Yb7Se12 crystallizes in space group Pnnm of the orthorhombic system with cell dimensions a = 12.385(4) Angstrom, b = 25.688(7) Angstrom, c = 4.063(1) Angstrom, and Z = 2. CsEr3Se5 crystallizes in space group Pnnm of the orthorhombic system with cell dimensions a = 22.059(9) Angstrom, b = 4.095(2) Angstrom, c = 12.155(5) Angstrom, and Z = 4. The structures of these compounds have infinite one-dimensional channels. The channels consist of edge-sharing RESe(6) (RE rare-earth metal) octahedral layers and alkali-metal ions reside in the channels. The RESe(6) octahedra in both compounds are distorted with bond lengths varing from 2.752(3) Angstrom to 2.939(2) Angstrom. The result of magnetic susceptibility and resistivity measurement of Rb3Yb7Se12 showed that it is paramegnetic and is an insulator. Structural comparison between Rb3Yb7Se12 and CsEr3Se5 is discussed. A series of additional isostructural compounds Cs3Y7Se12 and Rb3Er7Se12 have been prepared, and structural diagrams of ternary selenides with respect to cationic radius ratios were drawn.