Single-stranded guanine-rich sequences fold into compact G-quadruplexes. Although G-triplexes have been proposed and demonstrated as intermediates in the folding of G-quadruplexes, there is still a.,debate on their folding pathways. In this work, we employed magnetic tweezers to investigate the folding kinetics of single human telomeric G-quadruplexes. in 100 mM Na+ buffer. The results are consistent with a model in which the G-triplex is an in-pathway intermediate in the folding of the G-quadruplex. By finely tuning the force exerted on the G-quadruplex, we observed reversible transitions from the G-quadruplex to the G-triplex as well as from the G-triplex to the unfolded coil when the force was increased from 26 to 39 pN. The. energy. landscape derived from the probability distribution shows clearly that the G-quadruplex goes through an intermediate when it is unfolded; and vice versa.