Template DNA, primers, dNTPs, Mg^2+ concentration, dose of Taq DNA polymerase and annealing temperature are primary factors to ISSR-PCR. With the genomic DNA of Eucalyptus leaves as material, the six factors were systematically analyzed. The result shows that the optimal reaction system of ISSR is the 20μL system containing 20 ng template DNA, 0.4 μmol· L^-1 random primers, 0. 15 mmol· L^-1 dNTPs, 2. 0 mmol·L^-1Mg^2+, and 1. 25 U Taq DNA polymerase; and that the optimized annealing temperature is 55℃, the PCR procedure is : pre-denatur...