OBJECTIVE: To investigate the criterion for source identification of gastrointestinal tumor based on the number of identical allele (IAn) and the number of matched STR locus with 2 identical alleles (A2) in Identifiler system. METHODS: One hundred and five pairs of gastrointestinal tumor samples and homologous normal samples (TN group) were genotyped with Identifiler system. The numbers of STR locus with genotypic alteration (STRGA) in each tumor were determined by comparing the genotype of the matched STR loci in each pair of samples. According to the limited distribution of IAn and A2, 16 different values of IAn was substituted into the published discriminant functions to obtain the cut-off values of IAn and A2 for source identification of tumor sample. Indices including sensitivity (SEN), specificity (SPE), accuracy (AC), positive predictive value (PPV) and negative predictive value (NPV) for distinguishing tumor from an unrelated individual or a full sibling of the patient were calculated. Concordance of the identification results based on the determined criteria and the definite facts were statistically tested with Kappa index. RESULTS: The total frequency of STRGA was 5.46%. There were 31.43% of the 105 tumor samples carried at least one STR locus with STRGA mutation. According to the Fisher discrimination rules, criteria I (IAn>or=23 and A2>or=8) and criteriall (IAn>or=26 and A2>or=11) meet the requirements of distinguishing tumor sample from an unrelated individual or a full sibling of the patient with tumor, respectively. SEN=0.971 0, PPV=1.000 0, PPV=0.891 9 and Kappa=0.923 5, when the criteria were used to determine the specified relatives. CONCLUSION: Criteria I and criteria II were powerful for distinguishing tumor sample from an unrelated individual or a full sibling of the patient with tumor, respectively, when the Identifiler system was adopted for source identification of gastrointestinal tumor sample.