This paper presents a low-temperature co-fired ceramic (LTCC) diplexer for 2.4/5-GHz applications. A simple schematic based on lumped-element lowpass and bandpass filters is proposed for realizing the diplexer. The lowpass filter is designed with a cut-off frequency of 3.7 GHz and three transmission zeros are generated to improve the out-of-band rejection. The bandpass filter is proposed with two transmission zeros locating at two sides of the passband. By using a complementary structure to combine the lowpass and bandpass filters, the LTCC diplexer is simulated with low insertion loss of smaller than 0.38 dB for 2.4 GHz application and 0.59-0.65 dB for 5-GHz application. The in-band isolation at the two operating bands are better than 20 dB, which is acceptable for many wireless applications. The presented LTCC diplexer has a circuit size is 1.0 × 0.5 × 0.35 mm3, which is very compact as compared to reported LTCC diplexers.