The performance of the line current differential protection relay is influenced by several things, including: measurement of current transformers, measurement of voltage transformers, communication media and their own relays. Problematic communication media can cause line current differential protection relay function not working properly. This study was conducted to evaluate the effect of optical fiber communication media on the performance of Line Current Differential protection relays. This study used two protection relay samples. Each protection relay was carried out two test simulations, namely RTDS (Real Time Digital Simulation) dynamic testing with direct communication interference simulation using Variable Attenuator and RTDS dynamic testing with indirect communication simulation using Multiplexer through communication provider network. Based on the simulation result, it was found that signal attenuation and changes in distance of communication lines affected the performance of Line Current Differential protection relays. Signal attenuation can cause the blocking trip function in the protection relay not working properly, while changes in the communication path affect the Line Current Differential protection relay trip time.