The basic step to optimize the properties of filler in composite is the interfacial interaction between the matrix and the filler. Irradiation, a novel technique, was used in this work to introduce a wide range of defects in Single-walled Carbon nanotubes (SWNTs) as filler prior to composite formation. The thermal stability along with phase transition behavior of an organic conducting polymer Poly(3-hexylthiophene) (P3HT) loaded with pristine and ion implanted SWNTs has been investigated using Thermogravimetry Analysis (TGA), Differential Scanning Calorimetry (DSC), and Electron Spin Resonance (ESR). Interestingly, we observed substantial improvement on the thermal stability and the phase transition behavior of the composite with pristine and irradiated fillers.