Abstract: Objective To deduce the calculation formulae of likelihood ratio for kinship testing with X-short tandem repeat （X-STR） typing. Methods With the identity by decent coefficient of different relationships, the joint genotypic probability of two individuals with specific genotypes was calculated as X value, and then the joint genotypic probability of two unrelated individuals was calculated as Y value; therefore, the likelihood ratio value（X/Y） was obtained. Results The calculation formulae of the likelihood ratio for different genotype combinations of the female-female, female-male and male-male genetic relationships were derived and verified by real cases. Conclusion The calculation formulae are simple and accurate to evaluate the likelihood ratio for two individuals’ genetic relationship with X-STR typing. The formulae provide the basic potential value for the difficult kinship testing with X-STR loci.

Key words: forensic genetics, X chromosome, parent-child relations, short tandem repeat, likelihood ratio