This paper presents the design technique and simulation of Duplexer for GSM 900 band applications using microstrip technology.Two band pass filters with unequal impedance are designed. One filter with the 890-915MHz band and other filter with the 935-960MHz. Then these two filters are combined together in parallel to act as a duplexer with the uplink frequency band as 890-915MHz and downlink frequency band as 935-960MHz.The simulation is done using ADS software. Next, tuning and optimization are applied to achieve the low insertion loss.The proposed duplexer is a proof of concept for realizing duplexer functions using microstrip technology. In general, duplexers are built using high quality factor cavity filters. However, to prove the concept, duplexer is fabricated using FR-4 material which is readily available in India.