A dynamic chromatic dispersion (CD) compensating technique for a 40 Gb/s single-channeled optical fiber communication system is demonstrated. The tunable dispersion compensator is composed of 2×2 optical switches, dispersion compensating fiber etc. For controlling the tunable dispersion compensator, a feedback signal is obtained by detecting the narrow band electrical power centered at 12 GHz, and the electrical power increases with the decrease of the accumulated chromatic dispersion. In experiment, the maximal response time of the system is 0.7 s. The compensating range and precision reach 81.55 ps/nm and 5.28 ps/nm respectively which can be further improved by adding the number of optical switches and reducing the length of dispersion compensating fiber. Experiments indicated that the dynamic dispersion compensating system can significantly improve the performance of 40 Gb/s optical communication systems according to the eye diagram before and after dispersion compensating.