We propose a 180-degree branch line coupler composed of two types of iris-loaded waveguides. The proposed coupler consists of two main transmission lines and branch lines with different electrical lengths. Optimal electrical lengths enable to achieve 180-degree output phase difference without additional phase shifters. The two main lines with different electrical lengths are realized by capacitive and inductive iris-loaded waveguides. We designed a proposed coupler in K-band. The size of the proposed coupler is almost same as the usual 90-degree branch line coupler. So the proposed coupler presents an advantage of compactness, in comparison with the conventional 180-degree coupler with additional phase shifters. The design results demonstrated a return loss of 22dB, an isolation of 25dB, a coupling response of 3dB+/-0.1dB, phase differences of 0deg+5/-1deg and 180deg+5/-1deg with a bandwidth of 8% in the K-band.