An all digital 256 QAM modulator including the carrier signal of nearly 10 MHz, was constructed. In order to achieve, the very high-speed operation of nearly 100 MHz is required. In this experiment, the very high-speed digital devices such as digital multiplier with delay time of 4.3 ns was employed. Excellent modulation performance was achieved with no adjustment. Roll-off spectal shaping filter using a new BTF circuits was constructed. The excellent transmission performance was obtained and the transmission capacity of nearly 20 Mbit/s was achieved. The results provide prospects for an all digital LSI-based high-speed 256 QAM modem for high-capacity Digital Microwave Radio.