In this paper, the authors describe the design of a digital single-side band amplitude modulator with 40 kHz output using high-performance digital signal processor for ultrasonic transmitters. The baseband signal is converted to zero-IF signal by using quadrature frequency converter. Then zero-IF signal is filtered by two low-pass filter and converted to output frequency by frequency quadrature up-conversion. It is implemented by using single fixed-point digital signal processor. Experimental result shows that the digital modulation function is well operated, and performance of side-band rejection is better than 50 dB isolation.