Ultrasonic harmonic imaging provides superior image quality than linear imaging and has become an important diagnostic tool in many clinical applications. Nevertheless, the contrast-to-tissue ratio (CTR) in harmonic imaging is generally limited by tissue background signal comprising both the leakage harmonic signal and the tissue harmonic signal. Harmonic leakage generally occurs when a wideband transmit pulse is used for better axial resolution. In addition, generation of tissue harmonic signal during acoustic propagation also decreases the CTR. In this paper, suppression of tissue background signal in harmonic imaging is studied by selecting an optimal phase of the transmit signal to achieve destructive cancellation between the tissue harmonic signal and the leakage harmonic signal. With the optimal suppression phase, our results indicate that the tissue signal can be significantly reduced at second harmonic band, whereas the harmonic amplitude from contrast agents shows negligible change with the selection of transmit phase. Consequently, about 5-dB CTR improvement can be achieved from effective reduction of tissue background amplitude in optimal transmit phasing.