In this paper, a novel method of in-line chromatic dispersion (CD) measurement is proposed, we theoretically and experimentally demonstrated the CD measurement of 10-Gbit/s NRZ and RZ signals by a novel RF spectrum phase detection technique, this is performed using in-band tone monitoring RF, electrically down-converted to direct current (DC) or a low intermediate-frequency (IF) of less than 1MHz through electronic mixing with local oscillator (LO) of 2.4 GHz. The measurement provides a large CD measuring range with good accuracies (±2000 ± 35 ps/nm), and independent of the bit-rate and data format. In addition, the use of electronic mixer and low-speed detectors makes it cost effective for in-line CD measurement.