We investigate the relationship between the eye-penalty and the clock amplitude of the transmission signal. The simulation results for return-to-zero (RZ) signals show a good correspondence between the clock amplitude and the eye-penalty. For a non-return-to-zero (NRZ) signal, however, the correspondence between them is satisfied within the reasonable range of the transmission distance. Thus, we can monitor the eye-penalty by measuring the clock amplitude of the transmission signal. The proposed scheme may provide a cost-effective monitoring of the chromatic dispersion-induced signal distortion in optical transmission systems and networks.