This paper investigates radiated electromagnetic interference (EMI) causes and solutions in high-frequency active-clamp flyback (ACF) using GaN ICs. GaN devices enable high-frequency operation which shrinks the passive component size and can potentially be more efficient. However, the increased spectrum of radiated EMI is a concern with a lack of readily-available solutions. This paper analyzes noise mechanisms by studying waveform and spectrum compositions, identifying major noise contributors. Near field couplings of the cable-converter electric coupling can bypass the CM choke, inject noise current and cause the radiated EMI. This paper develops a model to characterize the near field coupling effect on the radiated EMI. Noise reduction techniques including a shielding and grounding technique, a CLC pi-type CM filter are proposed and verified. Experimental verifications are performed on a GaN IC-based ACF power adapter and radiated EMI is brought into compliance.