In this paper, the stepped impedance resonators are used to design a diplexer for the upcoming fifth generation (5G) millimeter-Wave wireless technology. The proposed diplexer design has two output ports, one port for 28 GHz band and the other port for 39 GHz band. The simulated reflection coefficient (S11) is -19.979dB at 28 GHz and -19.618dB at 39 GHz. The transmission coefficient is -0.67dB at 24.25GHz for the low-band section of diplexer and -0.419dB at 37GHz for the high-band section. The rejection level is -25.076 dB at 37GHz for low-band section and -24.344dB at 24GHz for high band section. The total circuit size is 17 x 6 x 0.14 mm3. The design is simulated using CST microwave studio and fabrication is in progress. © 2019 IEEE.