The complex formation between anionic polyelectrolyte, poly(acrylic acid) and cationic surfactant, tetradecyltrimethylammonium bromide (TTAB) has been carefully investigated by using isothermal titration calorimetry (ITC). The titration of TTAB into 0.05wt% PAA aqueous solution (molecular weight = 450000) demonstrates an exothermic peak due to the formation of PAA/TTAB complex driven by the electrostatic attraction between oppositely charged PAA and TTAB. When the TTAB concentration reaches a saturated concentration Cs, PAA is saturated with PAA/TTAB complexes. When the TTAB concentration is larger than Cs, TTAB would not bind to PAA anymore. When the TTAB concentration is further increased up to the critical micelle concentration Cm, the TTAB free micelles start to form. The effect of the PAA concentrations on the characteristic concentrations Cs and Cm is examined by using different concentrations of PAA aqueous solutions. It is found that the characteristic concentrations Cs and Cm increase along with the PAA concentration. The effect of PAA molecular weight on the aggregation behavior is explored by using four PAAs of different molecular weights: 450000, 250000, 25000, 5000 and 1800. As PAA molecular weight decreases from 450000 to 1800, there is no way to identify two characteristic concentrations Cs and Cm from the ITC measurements.