The broadband high speed digital signals may affect nearby wireless communication devices and cause significant electromagnetic interference (EMI) problem. Common solutions, such as shielding or layout improvement, usually focus on the coupling path control. This paper presents a new coding method compatible with 8b/10b to reduce EMI at the source. The coding uses the control-symbol K29.7 in 8b/10b to achieve spectrum suppression at specific target frequency. One or two K29.7s may be added into the data stream for every 4 to 10 data-symbols. The arrangement that has the smallest Fourier transform amplitude is selected as the final output signal. At the receiver end, these redundant K29.7s are simply discarded. The normal data transmission is not affected except throughput reduction. The only requirement of this coding scheme is that one specific control-symbol must be reserved for EMI reduction purpose, but not for other control function. It is found experimentally that the amplitude spectrum of a 5-Gbps 8b/10b source may be suppressed at 2.437 GHz by 7 to 12 dB, depending on the specific coding detail, which verifies the effectiveness of the proposed coding method.