In this paper, it was explained that IEEE 802.11b WLAN system, communication method, and throughput measurement of the EI. Next, we measured the throughput of the WLAN influenced the CCI and the ACI. And we obtained the threshold value by analyzing measurement result. Finally, under the method of Direct Sequence Spread Spectrum (DSSS) in WLAN, it explained that why the throughput decreases and why interference magnitude is different when Shift Frequency (the changed center frequency of the interference signal with the gap of 5MHz) changes from the despread process.