The hydrophobic interactions between poly(N-isopropylacrylamide), PNIPAM, and hydrophobically modified polyacrylamide, HMPAM, have been investigated by means of the polymer solvent viscometry method in dilute solutions. When HMPAM aqueous solution is used as a solvent, a divergence from linearity between the reduced viscosities of PNIPAM (as a solute) and C-PNIPAM is observed. While PNIPAM aqueous solution is used as a solvent, the linearity is better and the intrinsic viscosity of HMPAM is higher than that in 0.1 M NaCl. The magnitude of mutual interaction parameter K-AB suggests the presence of the hydrophobic interaction between HMPAM and PNIPAM, which is dependent on the hydrophobic groups content of HMPAM and temperature. Especially, as temperature increases from 25 to 30 degrees C, the degree of hydrophobic interactions is enhanced, as is reflected in the Huggins coefficient K-H of HMPAM increase from 1.954 to 7.987.