The impact of multipath interference (MPI) on the noise figure of fiber Raman amplifier is studied. The noise figure is analytically expressed in terms of the Raman gain and the Rayleigh scattering parameters of the Raman fiber. The difference between the power spectral characteristics of the multipath interference noise and that of the Raman spontaneous emission noise is considered. It is demonstrateed that the MPI can be a significant degrading factor to the noise figure when the Raman on-off gain and the input signal power are increased. A modified noise figure measuring method is proposed so that the noise figure accounts for the MPI deriving from the sum of Rayleigh backscattering signal components.