The phase noises of two narrow-linewidth fiber laser and laser diode are measured by using unbalanced Michelson interferometers with various optical path differences (OPDs). The measured results indicate that the phase noises of the two lasers do not change linearly with the OPD over the range from 1 to 100 m. The laser diode exhibits phase noise levels higher than that of the fiber laser at OPDs longer than 10 m. However, the laser diode outperforms the fiber laser at OPDs shorter than 10 m. The results obtained can assess laser performance and determine the suitable laser for use in a particular application.