We report on measurements of fractal dimension for ultrathin cobalt films deposited onto silicon substrates. A new method has been used for the fractal dimension measurements, which involves an image processing of transmission electron microscopy (TEM) bright field image of the ultrathin film structure. The TEM image is digitized by 512×512 pixels with intensity levels from 0 to 255, and topographic contour lines which connect the same intensity levels are obtained from the digitized image. The fractal dimension (D) of the ultrathin film structure is calculated from the contour lines by using a relation (L=AD/2) between the areas (A) and perimeters (L) for each of the closed lines as suggested by Mandelbrot [FractalGeometryofNature(Freeman, New York, 1982), Chap. 12]. Results of the measurements indicate that the Co‐Si interfacial dimension is increased from 2.0 to near 2.5 as the reaction is progressed. The results are compared with the fluctuating fractal dimension calculated from the noise exponent data.