20-Gbps non return-to-zero (NRZ) — binary phase shift keying (BPSK) using the silicon Mach-Zehnder modulator is demonstrated and characterized. Measurement of a constellation diagram confirms successful modulation of 20-Gbps BPSK with the silicon modulator. Transmission performance is characterized in the measurement of bit-error-rate in accumulated dispersion range from -347ps/nm to +334ps/nm using SMF and a dispersion compensating fiber module. Optical signal-to-noise ratio required for bit-error-rate of 10^-3 is 10.1dB at back-to-back condition. It is 1.2-dB difference from simulated value. Obtained dispersion tolerance less than 2-dB power penalty for bit-error-rate of 10^-3 is -220ps/nm to +230ps/nm. The symmetric dispersion tolerance indicates chirp-free modulation. Frequency chirp inherent in the modulation mechanism of the silicon MZM is also discussed with the simulation. The effect caused by the frequency chirp is limited to 3% shift in the chromatic dispersion range of 2dB power penalty for BER 10^-3. The effect inherent in the silicon modulation mechanism is confirmed to be very limited and not to cause any significant degradation in the transmission performance.