The most plentiful component of exoskeletons in invertebrates such as crustaceans and instects is chitin, which is the second abundant naturally occurring biopolymer by living organisms after cellulose. And Chitosan, one of the most important derivative of chitin, is an appealing biopolymer for the replacement of synthetic plastic compounds and applicable to various biomedical fields. Here, we measured molecular interactions of high molecular weight chitosan (MW ~110k, HMW chitosan) and low molecular weight chitosan (MW ~5k, LMW chitosan) in aqueous solutions by exploring the effects of pH, contact time and degree of acetylation which determines chitin and chitosna using a surface forces apparatus (SFA). SFA is a powerful instrument used to characterize the fundamental intermolecular forces such as electrostatic forces, van der Waals forces, capillary forces, hydrophobic interactions, bio-specific interactions, metal coordination forces as well as friction force in aqueous conditions. The nanomechanical studies of chitosan and chitin using SFA will provide new insight into the development of chitosan/chitin based load-bearing materials and biomedical applications.