This article presents a novel rectangular coaxial line with low cost in fabrication and low loss in propagation, which consists of inner conductor, outer conductor, and partially filled 3-D printing insulator support. The material used for the 3-D printing insulator is low-cost photosensitive resin in this work, and its relative permittivity and loss tangent are measured first. Then, a comprehensive study with guide-wave characteristics of a fully filled rectangular coaxial line is conducted. In addition, the relationship between the partially filled insulator and the transmission line performance as well as mechanical support strength is studied in detail. Compared with conventional coaxial line with fully filled dielectric support, the presented 3-D printing insulator support has its unique advantages of low cost and flexible shape, and reduced dielectric loss via partially filled insulator solution. After taking into consideration dielectric loss and additive manufacturing accuracy, the presented coaxial transmission line, three λ/2 resonators and a narrow-band third-order inline filter with two transitions at two terminals are characterized, manufactured, and measured so as to demonstrate the proposed concept. Herein, the insertion loss and the unloaded Q of the coaxial line resonator are used as the criteria for evaluating dielectric loss. The measured results of the designed coaxial transmission line at different temperatures show the stability of its performance. Good agreement among simulated and measured results validates the proposed approach.