Single molecule techniques have provided novel mechanistic insights on biological processes such as protein folding, transcription, and motor protein movement. Using single molecule methods, the distribution of individual molecular behavior is directly measured, which cannot be obtained using conventional bulk approaches. In this study, custom-built optical tweezers with sub-pN force resolution were used to probe the dynamic behavior of DNA:cationic carrier complex. Two histidine-lysine (HK) based polymers (H3K4b vs H3KG4b) were used to compare their condensation behaviors at the single molecular level. The difference between the two HK polymers at the single molecule level may have a significant implication as to why H3KG4b shows much higher gene delivery efficiency than H3K4b. The optical tweezers were also used to probe the unfolding processes of a fragment of F1 RNA. This can be used to characterize secondary structures in RNA, such as hairpins and pseudoknots.