High-speed connectors contribute to system-level electromagnetic interference (EMI). Radiation from antenna-mode current and transmission-line (TL)-mode current is briefly reviewed with a backplane connector and an optical cage connector. The radiation from the high-speed connector with TL-mode current is analyzed in detail. The possible radiation mechanisms are analyzed respectively through Green's function method, steepest descent method, and electromagnetic compatibility studio method of moment. Design guidelines of high-speed connectors for EMI mitigation are proposed based on the analysis of radiation physics.