The kinetics of the phase transition of poly(N-isopropylacrylamide) (PNIPAM) chains from the dilute regime to the semidilute regime was studied by using a homemade fluorescence spectrometer equipped with an ultrafast pulsed infrared laser (width similar to 10 ns and lambda = 134 mu m). We used 8-anilino-1-naphthalensulfonic acid ammonium salt (ANS) free in water as a fluorescent probe to monitor the conformation changes of the PNIPAM chains. Our results have revealed that in the dilute regime there exists two characteristic transition times that are less than 1 ms, namely, tau(pearls) (similar to 0.02 ms), which can be attributed to the nucleation and initial growth of some "pearls" (locally contracting segments) on the chain, and tau(coarsening) (similar to 0.2 ms), which is related to the merging and coarsening of the pearls. At semidilute regime, an extra (third) process (tau similar to 1.5 ms) appeared, and the contribution from the third process increased with PNIPAM concentration, which can presumably be related to the process of disentanglement of the polymer. A new method to detect the inhomogeneities of the polymer chain segments has been proposed.