Classification of seized methamphetamine by impurity profiling can provide very useful information in criminal investigations of drug traffic routes, sources of supply and relationships between seizures. The aim of this study is to improve and develop an analytical method for detecting impurities such as starting materials and by-products in illegally prepared methamphetamine.HCl samples. A 50mg sample of methamphetamine.HCl was dissolved in 1 ml of buffer solution (four parts 0.1M phosphate buffer pH 7.0 and one part 10% Na2CO3). Impurities were extracted with 0.5 ml of ethyl acetate containing four internal standards (ISs) (n-decane, n-pentadecane, n-nonadecane and n-hexacosane) and analyzed by gas chromatography (GC) using a flame ionization detector (FID) on a DB-5 capillary column (0.32 mmi.d. x 30 m, film thickness 1.0 microm). The use of a middle-bore column offered better separation of the impurity peaks. The correction of the retention times of impurity peaks with four ISs made peak identification very accurate for subsequent data processing. Twenty-four characteristic peaks were selected for comparison and similarity and/or dissimilarity between samples, and the data were evaluated by the Euclidean distance of the relative peak areas after logarithmic transformation. The results indicate that the present method would be useful for methamphetamine impurity profiling.