A scheme of quantum dialogue was proposed based on four-particle omega state. First, Bob, a communication party, prepared a four-particle omega state, encoded classical secret information by performing an approprite unitary transformation on two particles of the omega state and sent them to Alice. Then, Alice also performed an approprite unitary transformation on the two particles to encode secret information and sent back to Bob. Finally, Bob made joint measurement to final omega state, and announced the results. Alice and Bob, two parties of communication, could decode classical information the opposite one transfers, according to the announced measurement results and itself unitary operator, making the bidirectional communication come true. The result shows that the communication scheme only transfers two particles and makes unitary operations twice, and two communication parties get the opposite four-bit classcical information. It realizes both two-way communication of classical information with quantum channel and dense encoding to improve the communication capacity. The Eve′s intercept-resend attack can be found in time by inserting decoy photons in the system to raise the security of communication system.