A monolithic power splitter is used to split a pair of input differential signals into two pairs of output differential signals in the present invention. The monolithic power splitter has two input terminals to receive a pair of input differential signals, and it has two one-by-two power splitters integrated in one single chip to split a pair of input differential signals into two pairs of output differential signals with equal power. And, the monolithic power splitter has four output terminals to output two pairs of output differential signals. In one embodiment, the first one-by-two power splitter and the second one-by-two power splitter are made on the same surface of the substrate. In another embodiment, the first one-by-two power splitter and the second one-by-two power splitter are made on opposite surfaces of the substrate. The monolithic power splitter can be used as a power combiner based on the reciprocal property of the power splitter circuit.