Abstract: A nonlineax imaging technique is proposed that demodulate the second harmonic component of the response of the Ultrasound Contrast Agent (UCA). An UCA perfusion imaging method has been investigated from mathematical principle, simulation and ultrasound experiment. Coded excitation has been introduced to increase Signal-to-Noise Ratio (SNR) in medical ultrasound (US) imaging firstly. We performed a theoretical simulation and an ultrasound experiment to investigate if a cosine-enveloped Prequency-Modulated (FM) pulse excitation could be applied in US contrast agent imaging to increase SNR and Contrast-to-Tissue Ratio (CTR). To obtain a good imaging, there must be a right envelope on the transmitted signal and a gap between the transmitted signal bandwidth and the decoded match filter bandwidth. A harmonie image was obtained with the new coded excitation and new decoded filter. The results showed that this harmonie imaging method has good SNR and CTR. At the same time, the acoustical results were agreed with the theoretical results well.