Hydrophobically modified polyacrylamides are used as rheology modifier and thickener in various applications. Relationship between the microstructure and rheological properties of acrylamide/butyl acrylate (AM/BA) copolymers and AM/BA/2-acrylamido-2-methyl-1-propane sulfonic acid (AM/BA/AMPS) terpolymer synthesized by heterogeneous and micellar methods were studied in the present work. Effect of the polymer, NaCl and sodium dodecyl sulfate concentrations as well as of the temperature and shear rate on the aqueous solution viscosities of co- and terpolymers with different microstructures was investigated. Apparent viscosity increased significantly by increasing the BA-modified polyacrylamide concentration. It was attributed to the formation of hydrophobic BA nano-associations. Results showed that micellar samples with moderate molecular weight, multiblock microstructure and strong hydrophobic BA nano-associations have a rheological behavior comparable with that of high molecular weight heterogeneous sample with a random distribution of BA units. Among co- and terpolymers studied, the AM/BA/AMPS terpolymer showed the best thickening behavior against temperature and shear rate; however, thickening behavior was dropped significantly in the presence of NaCl. It was attributed to the presence of sulfonated AMPS anionic comonomer in the terpolymer structure. On the other hand, aqueous solution viscosity of the BA-modified copolymers increased by adding NaCl. It was attributed to the stronger hydrophobic BA nano-associations in the presence of NaCl. Results showed that molecular weight, hydrophobic BA nano-associations and the presence of sulfonated anionic comonomer in the co- and terpolymers have a significant effect on the aqueous solution viscosity.