In this paper, a series of compact power combiners are designed based on multimode transmission lines. Based on the multimode transmission lines, several multiport mode networks are constructed to achieve two-way, three-way, and four-way high-isolation power combinations. In detail, a four-port mode network based on a rectangular coaxial dual-mode transmission line and a five-port mode network based on a rectangular three-ridge three-mode transmission line are designed to achieve compact high-isolation two-way power combination. A six-port mode network based on a circular-coaxial three-mode transmission line is designed to achieve compact high-isolation three-way power combination. A nine-port mode network based on a four-ridge dual-coaxial six-mode transmission line is designed to achieve compact high-isolation four-way power combination. Based on a compact multimode transmission line, two-way, three-way, and four-way power combinations can be realized with single stages, high isolation, compact cross section sizes, low insertion loss, and high power capacity. The corresponding methods can be used to design several excellent power combiners.