The apparent adiabatic compressibility and the apparent molar volume of sodium perfluoroheptanoate (C6F13COONa) were obtained from the experimental data of ultrasound velocity and density of its aqueous solutions at the concentration above and below CMC (critical micelle concentration) as functions of applied pressure (0-70 MPa) and temperature (5-35 degrees C). Those were compared to the reported data for sodium decanoate (C9H19COONa) of Vikingstad et al. [J. Colloid Interface Sci. 72 (1979) 59]. It was found that the micelles of C6F13COONa were more easily compressible than C9H19COONa micelles under the condition studied. For the singly dispersed state below CMC, negative apparent compressibility of C6F13COONa was confirmed and it approached to zero with the increase of pressure or temperature suggesting the decreased electrostriction and hydrophobic hydration with pressure or temperature rise.