A 4X1 Multiplexing block for high speed signal processing was designed and fabricated using GaAs Buffered FET Logic. First, graphical design method was used to design the circuit roughly. And then, SPICE simulation was done to complete the circuit design. It was fabricated with mesa isolation & recess etching method. Each active device including MESFET, Schottky diode and active load represented good characteristics applicable to digital IC. And, the characteristics of the BFL inverter showed enough noise margin at low power consumption. But, output of the total circuit showed some difference with designed one because of the nonuniformity of the device characteristics on a wafer. If this problem be solved, fabrication of the multiplexing block which has uniform DC characteristics and can operate at high frequence will be possible with the proposed design and fabrication method.