Objective: Mutation of STR alleles has been studied aiming to avoid errors in DNA parentage assessment. Subjects and Methods: The study covers all DNA parentage testings occurred from may 4/2010 to march 8/2011; all cases happening with one inconsistency within STR allele between parents and child were studied for presence of mutation. Results: A study of 9586 paternal and maternal allele STR transferring to children, has showed 20 cases of mutation.