Human telomeric DNA, d(GGGTTA)n, tends to adopt complex intramolecular G-quadruplex structure in solution. The sequence folds into G-quadruplex with the antiparallel orientation in the presence of sodium ions, while antiparallel and parallel conformations were coexisted in solution containing relatively high concentration of potassium ions. Here, we describe that cationic comb-type copolymer, poly(l-lysine)-graft-dextran (PLL-g-Dex), triggers selective folding of d[GGG(TTAGGG)3] into G-quadruplex with the parallel orientation. The parallel G-quadruplex formed in the presence of the copolymer has the melting temperature > 90 degrees C, including strong stabilization effect of the copolymer.