The objective of this work was to apply an organic dye, tetraphenylporphrin (TPP), to probe the aggregation state, microviscosity, and diffusion dynamics of dye molecules within the interior microenvironment of conjugated polymer poly [(9,9dioctylfluorenyl-2,7-diyl)-co-(2,5-p-xylene)] nanoparticles (PFX NPs) which should open up further prospects in designing new porphyrin based nanoparticle materials and improve the knowledge of the better design of the nanophotonic devices based on dye-doped polymer NPs. This designed, aggregation-free TPP-doped PFX NPs system, exhibited remarkably high Forster resonance energy transfer (FRET) efficiency, leading to a potential application as a light harvesting system or a loadable drug carrier. To this end, different concentrations of TPP were doped into PFX NPs, prepared using a reprecipitation method. To compare TPP in organic solvent with TPP within the NP microenvironment, a successful doping of TPP into PFX NPs and no TPP aggregation formed inside of the NPs were indicated. Diffusion dynamics, location and degree of freedom of TPP in the PFX NPs were investigated using time-resolved anisotropy measurements fit with a "wobbling-in-cone and lateral diffusion" model. TPP molecules were found to wobble inside the core of NPs at a semiangle of ca. 70°and laterally diffuse on the NP surface with a diffusion coefficient of ca. 10 −4 cm 2 s −1 . The microviscosity of the nanoparticles was calculated from the wobbling model as 0.9−1.2 cP which is higher than the viscosity of THF (0.48 cP). By increasing the concentration of TPP from 0 to 5 wt %, the emission color could be gradually tuned from violet to red. Stern−Volmer analysis indicates that a single TPP molecule can quench 12 PFX polymer chains, and at 5 wt % TPP, PFX emission was almost completely quenched. The emission intensity of TPP in PFX NPs is 70 times higher than in organic solvent with the excitation wavelength at 350 nm which highlights the excellent sensitization properties of this system. The combined energy diffusion and Forster energy transfer model was used to simulate the relationship of nanoparticle sizes and the energy transfer efficiency. Both of the simulation and experimental results indicate that in this system, the FRET efficiency increases with the dye density increasing and they increase gradually for small particles range of 5−20 nm, approaching constant value for particle radii above 30 nm. The exciton diffusion length was estimated as 6.5 nm.