Short tendem repeat (STR) loci have been used in the field of forensic science. There are literally hundreds of STR systems which have been mapped throughout the human genome. These STR loci are found in almost every chromosome in the genome. They may be amplified using a variety of PCR primers. In this study, a DNA genotyping system based on the multiplex amplification of highly polymorphic STR loci was developed. Three STR loci with nonoverlapping allele size ranges have been utilized in the multiplex amplification including the Neurotensin receptor gene, D21S11, and Human tyrosine hydroxylase gene. The optimal condition for triplex PCr was obtained in a solution with a total volume of $25{mu}l$ containing 2.0 U of Taq polymerase, 3 mM of $MgCl_2$, $300{mu}M$ of dNTP, 10 pmole of each primer set, an annealing temperature of $62^{circ}C$, and 35 cycles. The optimized condition was successfully employed in a family paternity test.