A practical application of polarizing microscopy has been investigated by using key refractive indices (n1, n2) of group A drugs (lamellar, scales or plates). A graphic illustration of the retardation plotted log(n2-n1) as the abscissa and log(D) as the ordinate, where D denotes the thickness of the sample, gives a simple striped pattern composed of parallel straight lines having various retardations. As group A drugs have unique values of log(n2-n1), the thickness of a crystalline particle can easily be obtained from the graph by observing the interference color of the crystal under crossed polars.When the thickness of the crystal is obtained, the specific surface area can be calculated by the general microscopic method. This procedure could be useful for quality testing of poorly soluble drugs.