A more accurate analytical expression for the noise figure of fiber Raman amplifiers is derived. Then the impact of multiple-path interference induced by double Rayleigh scattering on the noise figure of fiber Raman amplifiers is studied. The results show that impact will be more serious when the fiber length, input signal power and Raman gain are increased. But the results also show that if the fiber length is optimized and Raman gain is properly controlled one can achieve the best trade-off between the noise and Raman gain.