A method for quantitatively determining the critical thickness, dcrit., for columnar growth of a CoPtCr-SiO2 granular layer was evaluated. The value of dcrit can be determined from a plot of the perpendicular magnetic anisotropy energy (Ku) of the layer, Ku × dmag., versus the layer thickness, dmag.. The value of dcrit reflects the structural uniformity of the layer in the thickness direction, and can be used as an index for choosing suitable deposition conditions and the material for the granular layer. Using the proposed method, columnar growth structures can be analyzed by evaluating their magnetic properties and layer thickness without observing cross-sectional TEM images. This inexpensive and simple method can potentially be used to characterize columnar growth structures on the nanometer scale.