We investigate magnetic and crystalline microstructures of melt-spun (Fe0.675Pt0.325 )100-xBx (x = 12, 14, 16, 18,20) nanocomposite ribbons after optimal thermal treatment using a magnetic force microscope. The magnetic microstructures are characterized by darker spots adjacent to brighter ones in a sub-micro scale and in random distribution. It is found that the strength of the exchange coupling interaction between the crystals in the 10-100 nm scale, implied by the maximum value (δM)max of the Henkel plot, could be roughly described by the ratio of the average width of the magnetic spots w to the average crystal size D for the ribbons. Moreover, we find that the intrinsic coercivity jHc of the ribbons is sensitive to their crystal sizes, and the smaller D, the higher jHc. Finally, by using roughness analysis, the curve of the root mean square values (δφ)rms of the phase shift of the magnetic force images versus the boron content x is obtained, which is qualitatively consistent with that of the magnetization σ12 koe of the ribbons versus x.