We produced the bimetallic Fe x Mo 100-x and Fe x W 100-x nanoclusters with cluster size of 1.5-4.5 nm by thermally decomposing a mixture of two metal carbonyl vapor species and recorded the X-ray diffraction (XRD) patterns of them for the mole percent (x) range of 0 e x e 100. The XRD patterns changed from that of pure M (M ) Mo and W) nanoclusters to that of pure Fe nanoclusters as x increased. The solubilities of bimetallic nanoclusters were 2-3 times higher than those of the corresponding bulk alloys, respectively. We observed that the cluster size decreased whereas the atomic level strain increased as the degree of alloying increased. We also observed that the cluster size effect on the broadening of the full width at half-maximum (fwhm) of the XRD peaks was stronger than the strain effect. Both cluster size decrease and atomic level strain increase of Fe x W 100-x nanoclusters were larger than those of Fe x Mo 100-x nanoclusters.