Poly(acrylamide-co-acrylic acid) [P(AM-AA)] has attracted many researchers for its special properties and potential applications in enhanced oil recovery (EOR)．The network-structure of P (AM-AA) in aqueous solutions and the behavior in 0.1 mol／L NaCl aqueous solutions were investigated by a combination of static and dynamic laser light scattering techniques．In the hydrodynamic radius (Rh) distribution of P(AM-AA) molecules in aqueous solutions

a size range of 100～500 nm

which is coincident with the network-structure

is dominant．With the addition of NaCl

the width of hydrodynamic radius distribution decreases

and the radii concentrate in a range below 100 nm．These results provide strong evidence of the existence of network-structure in aqueous solutions of P(AM-AA)．The network-structure will be destroyed with the addition of a small amount of salts

such as NaCl．and then the stability of the solution decreases due to the destruction of the network-structure．In 0.1 mol／L NaCl aqueous solutions

there exists a critical concentration c* (≈1.34×10-3 g／mL)：when c＜c*

the solution of P(AM-AA) is stable and without precipitation for long time；when c＞c*

white flocculent precipitates appear after a period of time．