We demonstrate rational harmonic mode-locking (RHML) in a very simple cavity Er-fiber ring laser. Using RHML the authors succeeded in otbaining the optical pulses at a repetition rate (fm) which is multiples of the RF drive frequency (fm≈1.4 GHz) with a detuning ratio of >10-6. The generation of the maximum frequency was about 6 GHz and the stable rational harmonic mode-locking occurs only at fb=2fm. The experiment shows that the RHML technique is feasible in a short and very simple cavity Er-fiber laser.