Investigation on compaction and decompaction of polymers is very important since it is a fundamental problem in polymer physics. With the aids of atomic force microscope (AFM) and dynamic light scattering (DIS) measurements in this contribution, the temperature-dependent compaction/decompaction transition process of water-soluble cationic polythiophene (PT) was investigated in the presence of la The above process is characterized by the red-to-yellow color change and fluorescence recovery and is reversible during the heating-cooling cycles in the range from 25 to 55 degrees C, indicating that the compaction and decompaction of polymer can be employed as a temperature indicator.