The interactions between monovalent counterions and polyelectrolyte are important in chemical and biological systems. The condensation and screening effect of counterions complicate the polyelectrolyte solutions. By means of single-molecule AFM, the singlechain mechanics of a strong polyelectrolyte, poly(sodium styrenesulfonate) (PSSNa), in KC1 aqueous solutions over almost whole concentration range have been studied. The M-FJC model has been used to describe the single-chain elasticity of PSSNa in KC1 solutions with a parameter of single-chain modulus (K-0). Along with the increase of the,concentration of KCl from zero to almost the saturation concentration, a reentrant variation of :K-0 of single PSSNa chain can be observed. When,[K+] iS between 0:01 to 3,M, the charges on the PSSNa backbone are almost completely screened, i.e., the PSSNa chain is virtually neutral in this case. Because K-0 has. a positive correlation with the net charge of the polymer. chain, the increased K-0 at very high KCl concentrations (>= 3.5 M) indicates that the chain is charged again. Due to the negative charges on the backbone of PSSNa, only the positively charged counterions (K) can be adsorbed on the chain. Thus; the PSSNa,Chain Should be positively charged When KCl concentrations >3.5 M. That is, the charge inversion,occurs in this,case, which, is - induced by a monovalent salt. This finding may lay the foundation for the future applications of drug delivery and gene therapy: