A new type of miniaturized microstrip diplexer is initially proposed based on the combined technology of fractal geometry and composite right/left handed transmission line (CRLH TL). The diplexer which is synthesized by combining two 3-port networks operating at a pair of different frequencies consists of lumped elements for the left handed (LH) part and Sierpinski-shaped microstrip lines (MLs) for the right handed (RH) part. Sierpinski curve allows miniaturized dimensions of the diplexer mainly attributing to its strong space-filling capability. CRLH TL enables dual-band design due to its unique hyperbolic-linear dispersion relation. For demonstration, three CRLH diplexers using Siepinski curves of different iteration orders are successfully designed. For verification, a CRLH diplexer sample by adopting a Sierpinski curve of second order was fabricated and measured. Consistent numerical and experimental results have confirmed the design. The fabricated diplexer operating at 0.9 and 1.8 GHz, respectively achieves good in-band performances and a maximal 76.3% size reduction compared to the conventional CRLH diplexer without fractal geometry.