Influences of stacking structure of aromatic layers and structure out of layers upon an XRD pattern were studied in order to establish the characterization method of size of aromatic layers in carbonaceous materials. According to the results with Warren-Bodenstein calculation, presence of stacking structure decreases the width of 11 band in XRD pattern, which could cause overestimation of layer size by our method derived from the Diamond method. It was also found that consideration of methylene group and biphenyl-like structure in chars was effective to raise the preciseness of our characterization method.