Short tandem repeats (STRs) have been widely used in forensic sciences such as stain analysis and paternity testing. Although most of STR typing could give the reliable and clear results, some unusual typing have been observed in forensic practice. The anomalous typing could result from a lot of causes, including DNA genetic variation, poor quality or quantity of DNA template, different typing system or method, nonspecific reaction in PCR or anomalous electrophoresis migration. The unusual results may disturb the right interpretation of STR typing.