A model is presented for relative intensity noise (RIN) in DFB fibre lasers which predicts the measured charateristics accurately. Calculation results imply that the RIN decreases rapidly with a stronger Bragg grating and higher pump power.It is shown that intrinsic loss gives servere limitations on the obtainable output power from a distributed feedback fibre laser and that the optimum coupling coeffcient will decrease when the cavity length is increased.