Tunable harmonic mode-locking of (HML) one kind of dissipative solitons (DSs) has been experimentally demonstrated in an Er-doped fiber (EDF) laser for the first time. The laser is mode-locked by nonlinear polarization rotation. The repetition rate can be tuned from the 2nd to 10th-order HML. A high repetition rate of 232 MHz (10th- order HML) has been observed in this laser, and the pulse duration after compression is 153-fs with single-pulse energy of 0.5 nJ. The spectral width of the pulses decreases as the harmonic order increases in the experiment, which is demonstrated for the first time, consistent with the numerical study. A rule based on monitoring the pulse spectral width to find desired-order HML is also proposed.