An analysis of STR DNA polymorphism examination results was presented in 45 biological samples and 29 samples of comparative blood. They made up biological evidence in 19 criminal cases. DNA was extracted from different biological samples (blood stains, semen stains and tissues) by using "Blood DNA Prep Plus" kit (A & A Biotechnology). Six STR loci were amplified in 2 triplex reactions (CTT and FFv) according to GenePrintTM kit (Promega). An amplification efficiency analysis was carried out. Concrete power of discrimination was calculated on the bases of genetic traits and data concerning the occurrence of these alleles in the population of south-east Poland for concrete STR systems of either triplex and both triplexes jointly.