Two new ternary Zintl phases, Sr5Sn2As6 and Eu5Sn2As6, have been synthesized, and their structures have been accurately determined through single-crystal X-ray diffraction. Both compounds crystallize in orthorhombic space group Pbam (No. 55, Z = 2) with cell parameters of a = 12.482(3)/12.281(5) angstrom, b = 14.137(3)/13.941(5) angstrom, and c = 4.2440(10)/4.2029(16) angstrom for Sr5Sn2As6 (R1 = 0.0341; wR2 = 0.0628) and Eu5Sn2As6 (R1 = 0.0324; wR2 = 0.0766), respectively. Their structure belongs to the Sr5Sn2P6 type, which can be closely related to the Ca5Ga2As6 type. Electronic band structure calculations based on the density functional theory reveal an interesting electronic effect in the structure formation of these two types of Zintl phases, which substantially affect their corresponding electronic band structure. Related studies on the thermal stability, magnetism, and thermoelectric properties of Eu5Sn2As6 are presented as well.